This is going to sound partisan, but I genuinely think it’s because Trump connects with people on more levels than the Democrats do.
The name calling is part of his blustery comedic Hollywood side, which people understand is different to his policy making side. Watch the all-in podcast episode with him if you genuinely want to see a different side to him.
In contrast, Democrats often come across as only having a singular serious facet to their personality, and so immaturity undermines their entire character.