For a long time, Republicans have had to win by more than the margin of Democratic cheating. That means every vote potentially matters.
Trump, and apparently many of his supporters, believe it is funny when he insults other Republicans in the primary. I mean, the polls right now show him with more support than all the other candidates combined, so why should Trump or his supporters care if he alienates those Republicans and their supporters? He's going to win the primary anyway, so what non-Trump Republicans think really doesn't matter.
Except that it may well matter very much if he gets to the general election. They'll blame the inevitable loss on fraud, the MSM, George Soros, and anyone else they can think of.
What they won't do is look in the mirror and consider how many potential GOP votes stayed home because of how he and many of his supporters behaved, what they did to the rest of the GOP, and how they made the GOP look in the eyes of swing voters.