You're going to get a LOT of reductive and low effort answers from Lemmy radicals. But this is a super complex question, and there's not a 5-second ELI5 answer if you really want to understand.
Also, when the radicals scream at you, there's going to be a core of truth. They're going to yell about colonization and empires. That's a major factor, but not an exclusive one. However, for getting radical and rabidly furious its all they'll bother posting to you.
Things to investigate, because answering this for yourself in a meaningful way is going to take a while and require study. Here are some topics but NOT an exhaustive list:
Colonization
Resources (natural and otherwise)
Schooling, education, etc.
Stability, politically and otherwise (note this will have overlap with colonial and non-colonial powers destabilizing things intentionally for geopolitical gain)
Medicine as regionally practiced, traditional vs based on the the scientific method.
Geopolitics (isolationism, etc)
Geography (i.e. the US's greatest asset is its location, it neighbors no enemies and its main enemies are separated by an ocean. One of the key reasons the US focuses on the ability to project force)
Religion
Corruption (politically and non politically)
Crime and non-military/nation based violence (also could get grouped under personal safety and security)
And again, honestly, a lot of these topics will overlap, but that's what I mean by there isn't a quick, easy answer.
And the reductive stupid answer is just yelling colonialism.
There's a reason people get PhDs in this subject. It's not a quick, easy question.
In this topic: people who underestimate the importance of infrastructure and low crime and low corruption.
1st answer: developing countries don't have enough infrastructure to benefit from wealth. Not enough trains to move raw goods around, not enough roads or not enough electricity to do anything even if those good arrived.
2nd level: when governments get the money for such projects, they steal it from the people through corruption. See Turkey and all the invested dollars on earthquake-proofing buildings, it was all stolen in ways people didn't understand or realize until the earthquake happened.
3rd level: even if the government didn't steal the money, criminals can. Even in the USA we deal with transformer thieves (transformers are bundles of copper that convert long distance high voltage power into short distance power for houses). These copper bundles can sell for $$$$ in the black market.
So even if #1 and #2 miraculously happen, a criminal will steal the infrastructure and they gotta start all over again.
Everyone knows how to make cities more advanced and better. Build highways, trains, mass transit. Invest into freight (trains or boats). Invest into education so that people can run these machines.
And many 3rd world countries advance forward. But it's harder to do than it looks.
Everyone seems to be focusing on colonialism, but that really only brought Europe to a standard of living near India and China.
The real major thing that happened was that "the West" started industrializing before the rest of the world did. Some of the wealth came from colonial holdings that industrial countries had, but a lot of it came from having citizens who were more than a order of magnitude more economically productive than citizens of other countries for over a century.
Why the Indian subcontinent and China didn't industrialize at the time is up to debate, but some theories are related to lower labor costs not sparking the positive feedback engine of industrialization until it was too late to compete against the West and going into periods of relative decline that Western countries could take advantage of.
The West was able to make itself the factory of the world, pushing the rest of the world into resource extraction.
It wasn't until after World War II that other parts of the world were able to industrialize.
1 The middle income trap. Many countries used their cheap uneducated population as an opportunity for cheap labour, for large companies. This brings lots of capital to the country and people, and the country develops. Building more schools, infrastructure etc. but as a country develops, pay increases for workers, and suddenly their labour is no longer cheap. Their country's economy is now effectively stuck.
2 Conflict and instability. Investors don't want to pour money into a country where it might have a coup, leadership change, etc. They don't want to lose what they invest, since these events usually result in lots of private property being taken or destroyed. This fact leaves a lot of countries in a catch 22. They need investment to stabilize, but need to stabilize to gain investment.
A lot of countries are also unstable because of badly drawn borders. This often leaves a lot of ethnic tensions that continue to boil away indefinitely. Sometimes the borders give a country horrible geography and incentivise them to invade their neighbors.
One example would be that country #1 is downstream of a major river, behind country #2 and #3. Country #2 and 3 use a lot of the water and there is none left for country #1 and their only option is to invade.
The final and probably most common reason is that dictators don't care about prosperity, and that dictators lead to more dictators. Far more often than not, coups lead to another, worse dictator, focused on holding power than their country's success.
The reason that south Korea and Taiwan are successful and democratic today are because they rolled the 1/1000 chance on a benevolent dictator that WILLINGLY steered the country into democracy and genuinely improved the economy.
There's a lot but it mainly comes down to how Europeans were more developed than the rest of the world due to their frequent wars, so when they went to colonize the world nobody stood a chance. And since colonialism and the subsequent horrible decolonization messed up those countries, we get the state of the world today.
To be more specific, colonialism basically turned affected countries into oversized plantations run by foreigners. Any political development that was already there went out the window, and of course no more could be made. Then you got decolonization, where you had countries either being fought off (like France) or packing their bags and leaving (like the British). This created massive power vacuums, and when you have power vacuums you get power struggles and dictatorships and from there we see the world's current state. On the other hand you have Botswana, where they actually had a native ruler class who could rule when the British left. They were an occupied country, not an oversized plantation, so they're virtually one of the best places to be in Africa. Also specifically in Africa colonies would have their borders drawn with no care for the relationship between the people living there, and occasionally they'd actively set them up for failure by putting rival ethnic groups together.
And of course you have neo-colonialism and shit that even now continue to hold back African development.
TL;DR: Europeans came, turned everything into a plantation, then when they left the plantation collapsed and either a dictator came or things returned to survival for the fittest which then produced war-torn dictatorships. These countries should be able to become decent countries with time, and there are many examples of that happening, but the West is still preventing it in many places. See: France in Africa, cold war-era US in Latin America.
Of course we can get into infrastructure and education and all that, but all these things have their roots in the simple fact that these countries had a horrible start (whether a civil war or a dictatorship) and in many cases had to build states from scratch, and in politics a bad start can cost you decades.
Extractive capitalism pulls resources and wealth out of "developing" nations, leaving them poor. Power and money maintain power and money through a boot on the throat militarily and economically and by fomenting internal conflict within the "developing" nations.
Mostly corruption and stability doesn't allow for business to develop along with the wealth that brings.
There are other factors but mainly you need good governance and free markets to allow for wealth creation. It at least that is the only model that has worked so far.
Look up 'Elite Capture'. It's really hard to build good institutions and keep them strong and free from corruption, and they will be under siege by special interests from day one.
It's only my interpretation of it, so be wary. My idea is that after ww2, USA was terrified of USSR, so they did their best to avoid countries "falling" to it.
This best was of two categories: if it was an old power, feed it with all possible money, so they can can develop an industry to get all of the modern commodities (home, car, a fully equipped kitchen...)
If it was a colonised or USSR friendly country, forbid all trade, and feed civil war with all means possible, so that this country stop being communist.
Then, democracy had that people had to be listened to a bit, or they would vote communist. Car industries were favoured because it can be converted into a war industry if it needs. Roads and trains are also war assets. Healthcare and food are priorities to make people happy. Education and research are priorities for any country that want to stay relevant, and these benefit from co-operation with other countries.
The way I see it, the west built solid infrastructures and invested in the people in order to fight USSR, while USSR progressively fell into an oppression that prevented these progresses. The third world countries were left alone because no side would allow them to join the other side.
Now the world is full capitalist, so no one will invest in the countries that were left behind. With less investment they progress more slowly.
Riches have high standarts of living. Poors have no life, they survive. "Developed" countries has more middle class than others, which are promised to be rich by rich if they help rich to get richer by stealing from poor by capitalism.
I think it is because of population vs resources allocated per person. When a nation is developing it is still trying to catch up with the high number of population it can service, but with little resource it can utilize or there is but not yet utilize. It has no choice but to cut corners in turn lower standard of education, health, social services, housing and unutilize laws. This in turn having some or majority of the people receiving less and some none at all. This makes them vulnerable to bad influence and bad decision e.g. vote buying, rebellion. They cannot participate in the nation building process in a right mind since they are trying to survive. Anyways, I'm probably just talking bullshit. To be fair not all Western nations have high standard of living e.g. some nations in eastern Europe.
I think a big factor is the western labour movement wrestling away some of the prosperity created by the industrial revolution. Developing nations have profitable industries but the wealth doesn't make it's way down to the average citizen because they haven't forced it to happen. The small minority of people who do profit from dirt cheap labour are quite happy for things to stay that way indefinitely, and so it does, because they are the ones who hold political and financial power.