Because arts uplift, and misery is a huge part of the engine of fascism
Well adjusted societies have less temptation for it on the pro side, and a lot stronger and more inspired resistance to it from the anti side. It needs to breed sickness in order to really take hold and thrive.
I doubt DeSantis has that much of an organized plan for it, of course; I think it’s simple mean spirited ness on his part that he happened to see some vulnerable people that he can punish.