Imagine you moved countries, and were entitled to vote in both.

You have to tell the new country you exist there.

That’s the most common failure mode in the US, when you move states or even counties and there’s a miscommunication or lack of communication between where you came from and where you are. There is no top level federal voter database.

There are other issues, but this is the most common.

You don’t vote at a federal level, you vote at a state level, for federal stuff. (And state/local stuff)