What explains the difference between heating systems in the USA?

The oldest areas on the east coast were settled before gas lines were a thing and electricity existed. So they use oil unless it’s a big city that paid to have gas lines installed.
This is why cities that grew a lot after gas was industrialized primarily have natural gas.
Remote areas will use oil, propane, or wood because they can be delivered by truck and heat pumps are a pretty new technology that hasn’t worked well in cold areas until recently.
The south has minimal heat requirements so they can get by with electric which is cheap to install but not efficient enough to provide primary heat in cold areas.
Also, southern homes generally have air conditioning so adding a reversing valve or set of heater coils is pretty easy.
- The answer boils down to: Availability 🌈

One reason is bad laws. I would love to replace my natural gas boiler and steam radiators with an electric heat pump system. It would be more efficient, cheaper to run, be able use solar power, and keep my house more comfortable.
But state law requires that buildings meet a specific efficiency requirement, which my house does not meet because it is 170 years old. I understand why they have that requirement for new construction, but it is stupid to not have an exception for old buildings.
- I was looking at what I'd need to get a heat pump installed yesterday, before the state website even started discussing the different types there were links to weatherization programs and efficiency requirements. My house is about 70 years old, even with insulation updates in the 00s it's a drafty bitch
  
  Our home is from the 19th century, but with modern insulation it's still fit to be heated with ACs and a heat pump. We've opted not to since that's a big ass unit and we're limited on space, but it could work.
  We did have to replace isolation basically everywhere and we still need to replace some door frames, but the total cost was just a few thousand euros, before govt funding.
  Doing almost all of the work yourselves saves a fuckton of money.
- Did you look into it though? Anything you can do to weatherproof your home is far cheaper than paying forever to heat or cool it.
  I’m in a similar situation but less so. My house is only half the age but well built so not too leaky. My state strongly encourages efficiency measures and they’re not a bad idea. However since their criteria is “recommended” insulation, I’m “ok”. Basically since I have two layers of fiberglass in my attic, I’ve “done all I could”. However I did have to have an insulating contractor out to say “yep, can’t do any more”
  Now my only problem was that I have functioning (but old) heating and a/c so wanted to save up a few years, but now I need to decide whether I have to try asap to get the incentive while it lasts

Climate. You're not going to need gas heating in Florida when it's cold for only a short time of the year.
- Makes sense for Florida! I expected the opposite on the West coast though
  
  Electricity is more expensive out west than in the deep South. Also there's more oil and gas wells nearby lowering the transport/delivery costs.
  
  Do people in Southern California even need to heat their homes? Even in the Pacific Northwest close to the ocean (like Portland/Seattle) it doesn’t get below freezing much unless you’re at a higher elevation, so electric can still be adequate.
- Electric heating also works great basically everywhere else, too. Actually climate has very little to do with it in most cases and if there’s any non-manipulative reason(gas companies are really bad for this) it’s to do with what infrastructure makes the most sense.
  In places like Québec we have electric heating because hydro makes electricity pretty cheap. Up north in Nunavut they will often have big tanks on the property and a truck will come by to refill them.
  
  Infrastructure definitely contributes to what's going to be used. Good call.
- I thought at least south Florida had no need for heating.
  
  I’ve been told that not only does it occasionally freeze in Florida but that houses were essentially uninsulated. Before a/c, there was no reason to insulate in the south.
  Electric heat also has the advantage of nothing to maintain. You can have your baseboard heating just sit there most of the time, but it will still come on the rare times you need it

Age of the city and sufficient infrastructure. Once the building is built to use [oil or gas or whatever] it’s never refit to use something else. Something something quarterly profits.

Market supply and demand I assume.
Age of home probably plays a factor as well.
- Why would the age of a hole change it's preferred heating method?
  
  Meant home.
  If a house is setup for boiler/oil it’d be a real pain to retrofit it got forced air heating (nat gas, electric)

Probably also a bit of a history component.
When my parents built their house in rural New York, they could have chosen gas or propane. However that was the height of the first wave of nuclear power, when it was being promised “electricity will soon be too cheap to meter”. Of course now that looks like an insane choice as the most expensive.
Any areas built out during the 1950s-1980s may have followed this logic

Natural gas here, because the city has an extensive network. Previously, living with my parents a city away, the condo had oil heat.
A year and a half ago I had to replace my cast-iron boiler and (leaking) hot water heater, and so I took advantage of financing to replace them with a gas-powered tankless combo.