Because voting regulations are left to the State governments, and each state government does it slightly differently, often with designs that are specifically intended to disenfranchise specific voters.
Further, because of the Electoral College, it is very important WHERE you vote. If I live in New York, I can't vote in Pennsylvania. I get lumped together with everyone in New York.
So my registration ties me to a "permanent address" that aligns with a state, their electoral college contribution, and the rules they've put in place to gather, validate, and verify the vote, all mixed with manipulation over the years to swing the vote wherever possible (see: gerrymandering)