Wild speculation: republicanism only exists as a cult created by the propaganda of a political party. Of course one of the core tenets of such a cult would be voting above all else. The Democratic voters are all the other, non-cult members who don’t have any such religious fanaticism about voting
I suspect it has something to do with the collapse of the new deal coalition and the rise of neoliberal economics such as trade deals meant to gut unions, finance and media consolidation, and ending the welfare state...
As late as 1980s, the GOP had a Liberal wing. Once Reagan got his claws firmly embedded the Right was able to marginalize or remove them.
There's an old saying in US politics; Democrats fall in love and GOP fall in line. GOP will vote for any candidate with an R next to their name, and the Dems want someone they can get excited about.