Because campaigning works differently across the globe. So does media coverage and politics and how the different parts of legislative share competence/responsibility.
I'd say you need to follow local news and what happened in the last years to make an informed decision. See which party/politicians built children's playgrounds, did something useful for the community and which major tore down the shops in the city center to make space for more lucrative office buildings... It's really difficult to tell if you don't know what's happening around you but just following world politics. (And local newspapers are long gone, so that's another hurdle.) Where I live they have some scarce information online. And you can also go to them once they go campaigning and listen to them or read some advertising material.
A big part of my decision is the party they're part of. At least when I never read their names before. I'm also not affiliated with one specific party, but I certainly know if I'm conservative, religious or progressive and whether I care for the environment and other people. So I can base my decisions on that and whether their party cares for the things I deem important.
It varies everywhere, even from state to state in the US.
The US system is kind of broken - they ask you to vote for way too many things. Where I'm from I just vote for a party - I basically say "yeah, the green party are cool", and then the party decides who to put in which position should they get enough votes. I can give a +1 to candidates I like personally, but I don't have to.
In the US you might be asked to vote for school boards, a sheriff, and a bunch of weird positions. There's no realistic chance you'll make an informed decision for all of them.
Sadly, it's very important you still vote, because the republicans are using this broken system to fill these positions with far-right lunatics. So basically seek out information as much as you can, but at the end of the day just vote for whichever Democrat is on the ballot whenever in doubt. They're not guaranteed to be good - in fact they're likely to be pretty bad - but they're pretty much guaranteed to be the lesser of two evils.
Still might vary though - local politics are weird, and there are no rules set in stones. Some places you still have decent republicans on the local level (or so I've heard).