a) Republicans did the math and figured out that a lot of people who vote Democrat didn't have IDs. Old black people, in particular, were not likely to have IDs. Republicans also did stuff like accept fishing and hunting licenses for voting, but not university ID. None of this is a secret. A think-tank called ALEC came up with it. In-person voter fraud, the only kind of election rigging that voter ID laws prevent, is next to impossible in the US system and basically never occurs on more than a one-off basis.

b) A lot of those people who didn't have IDs have either gotten IDs or died off by now, so the Republican advantage of voter ID laws has faded.

c) Given (b), Republicans have moved on to other tactics like voter purges, shutting down registration offices and polling stations to create long lines in urban districts, gerrymandering, and limiting early voting and mail in voting. One of their favorite tactics is limiting early-voting mailboxes to one per precinct, whether that precinct has 1,000 voters or 1,000,000. You can guess which way the crowded urban precincts tend to vote.

The whole idea is just to put their thumb on the scale enough to discourage some small % of voters in the swing states that determine our president every four years. If you live in California, no one cares how you vote for President.