It's because the political coalitions that make up the democratic party are far less singular than the ones that make up the Republican base. There is essentially nothing that unifies the democratic base other than being anti evangelical and you can't really say that outloud as black evangelicals are also a part of the Dem base.

The Republicans aren't really anything more than just the evangelical christians movement these days alongside doing whatever any sort of willing donor is willing to give them as it relates to any other political issue.