Because hes an Ego maniac. If someone called him a "poopy head", I would expect him to throw a temper tantrum and purge them. . . . Hes like a 70 year old 3rd grader. Draining the swamp is pretty much the only thing we could believe him on.
*edit*
I Never took the phrase "Draining the swamp" to mean, get rid of life long politicians, I took it to mean "everyone who talked shit about me is going to be looking for a new job".