Because to two reasons in my opinion: Firstly, the people who are backing him are swamp creatures, Secondly, because he did not give Trump any thanks for helping him become Gov. He could have been more gracious and supportive.
http://www.youtube.com/watch?v=O8_pgBQdH3o
From first victory speech as governor, DeSantis thanks Trump at 6:41+
Sometimes it pays to do some research before asserting your opinion as truth when it wasn’t true at all.