Explain Like I'm 5 (ELI5)

Lot of dumb comments here. Everyone in a country with voting registers to vote, you just may not realize it. In order to vote in an election, the government needs to know that 1) you are a citizen, 2) you are alive, and 3) where you live so they know you can vote in that local district.

Ask yourself, if you moved across your country, how would you vote in those local elections? The answer is that you would register to vote there. You may not see it that way, cause that "registration" may be dual purposed with some other act (like getting a new drivers license), but you are letting the government know you can vote there. Most places in the US (at least everywhere I've lived, I think) allowed me to register to vote when I got my new ID.

The thing that makes it a little different in the US is that registering to vote isn't strictly tied to anything else. If you move from one city to another within the same state, or if you choose not to get a new drivers license in a new state, the government will have no clue where you live. You aren't required to give the government that information. The only time you have to give the government an address is to file taxes (maybe not even then). That doesn't occur until spring, so if you move in the summer, there's no way for the government to know that you can vote in a different place if you don't tell them.