Because they have made a deal with the devil and throwing in the towel means payment comes due. Thanks to the southern strategy Republicans hitched their proverbial wagon to the Christian extremists (who make up the bulk of the racists the southern strategy is predicated on). Decades of propaganda by the likes of Fox “News”, Rush Limbaugh, Tucker Carlson, Alex Jones, and Glenn Beck has left their party deeply mired with conspiracy theorists and people with only a tenuous grasp of reality, and an equal number of grifters and con artists preying on those same vulnerable populations. These people are the backbone of Trumps support and they have for various reasons completely embraced him in some cases even as a literal messiah. The Republican party is already struggling to attract enough voters to continue to win elections without resorting to voter fraud, they literally can not afford to give up a single voter, and turning against Trump at this point would shatter their party.

Trump is an existential threat to the Republican party, but one that perversely they can’t publicly attack without inciting the very thing that would destroy them. Like Frakenstein they’ve created a monster, and now it has turned on them. They’re frantically trying to keep its ire focused anywhere but on themselves and just hoping it does more damage to their opponents than it does to them before they can think up some way to contain or kill it.