Because of the electoral college, getting out to vote when you otherwise wouldn’t have is almost entirely useless in most states as the party most states will side with is basically a forgone conclusion.
On top of that, the Republican party has spent decades making it deliberately difficult to vote by limiting the number of polling places, as well as limiting mail in voting.
The Democratic party on the other hand refuses to attempt radical economic upheaval that poorer US citizens desperately need because Democrats are still the “moderate” party that is in the pocket of billionaires.
The Republicans routinely promise radical actions that they say will make things better for everyone. They’re lying of course, but it’s hard to be exceptionally poor, have no access to health care and vote for the party that is not advocating radical actions on your behalf.
Every election is held on a Tuesday. Most of those people cannot afford to take a day off of work because capitalism has fucked them over so badly.
Republicans have also stymied any attempts to turn election day into a national holiday specifically for this reason.