Primaries, primaries, primaries.
While David's answer is correct, any Trump run in 2024 would also remobilize the never-Trumpers so the overall advantage is uncertain.
However the US POTUS, and Congress, elections require candidates to first get past the hurdle of the most partisan voters of their party, be it Dem or Rep.
Right now, most really, really, committed Republicans back Trump. Simple as that. That's what all those "90% still back Trump" polls say, even as, even in places like Fox News comments - scroll to end, see up/down votes by stance, Jan 5th, 2021, his diehard supporters get pushback when they try to spin his election ramblings.
Comes a primary, chances are, unless the Republican party takes a hard stance against, Trump supporters are either going to vote Trump or whoever is backed by Trump. Anyone not anointed by Trump, from today's political calculus, risks being ousted. That may change over time, but it is a very real risk today. More exactly, anyone who gets targeted by one of Trump's infantile "So-and-so is a very bad man. Sad." tweets, or whatever mechanism he'll use to replace it, will get piled on by the faithful. And Trump is so irascible and over-sensitive that anything can set him off. Hence all the ring-kissing to avoid getting on his bad side.
As suggested by Fizz, here's an example of Trump turning against a Republican he didn't like anymore, Jeff Sessions, his erstwhile Attorney General, who'd lost his favor. Or Mark Sanford, who criticized him (and who admittedly had other skeletons in his closet).
A "hard stance against Trump", whether or not some Republicans would like it to happen, risks splitting the party and thus losing to the Dems. The Republican party seems roughly at 48% and can win, given electoral college and creative redistricting. It can't if 20% of its voters don't vote or back splinter candidates. Thus, a principled stance means losing at least one, possibly more, electoral cycles until the Trump voters cool off.
It's happened before. In 2016, anyone reasonably mainstream in the Rep party got primary-ditched early on and Ted Cruz, Mr. Abortion-is-wrong-in-case-of-rape was last man standing before Trump won.
Lest you see it as a Republican disease, from which the Democrats are immune, look at Sander's foray into the primaries twice, pulling the party leftwards, even as he's not even a Democrat.
Primaries motivate the uncompromising members of both parties. Right now, for Republicans, there is no one more uncompromising than a Trump who has lost. And his backers.
The one good thing in all this? January 6th, 2021 will forever be attack-ad-gold against Trump and his backers. By Democrats. By Republicans.
* And before you argue that he was just doing the principled thing with regards to Sanford, look how long it took him to ditch Roy Moore in Alabama.