Because they get their "news" from the regular "main stream" news-liars, and they are dumb enough to believe them.
The idea that people hate Trump is entirely the product of fabrications by the media-liar-system.
Republicans who are only concerned with "winning" and who take these assertions at face value, resent Trump because they believe the media lies that *HE* is the problem.
They are too naive to realize the media-liars will make up crap about anyone they see as a threat, and they will do the same to *ANY* other candidate, including DeSantis, whom they've already started to attack.
It’s their plan to make us fight each other. So far, it’s working but I hope never-Trump stops to think after the primaries.