In short, Trump does Trump. And, man, is it an entertaining show. Hell, he gives away free hats!
Meanwhile, the other 14 men and one woman running for the Republican presidential nomination function, effectively, as audience members at The Donald Trump Show. They can't (or won't) say or do the things Trump will and, therefore, always wind up on the short end of feuds with The Donald. Bush can spend all day attacking Trump's Republican bona fides, but it doesn't work nearly as effectively as when Trump calls the former Florida governor (essentially) boring.
That reality may change at some point — maybe soon. But, at the moment, every other candidate in this race is just a pawn in the Donald Trump game. Maybe they'll get lucky and get a hat!