Stephen Colbert and Jimmy Fallon approach politics and Donald Trump in very different ways. Fallon does a caricature, lots of jokes, but nothing nasty and pointed. Colbert, an unabashed liberal who greatly dislikes Trump, goes for the jugular.
Maybe it's what the late night viewers want? In the last two weeks, Colbert (remember the news stories that James Corden was going to replace him?) is now leading the pack. It's not that his show is so much more entertaining, but clearly people are turning in to her his monologue which is all about what's happening in The White House. Here is last night's monologue, followed by Jimmy Fallon, and you see the difference.