OK I’ll bite...
Because he is their worst nightmare, someone who is not a career politician, is too rich to be bought, (that would have happened long ago if it were possible and he would no longer be anything more but a footnote in the campaign coverage) and has the backbone to stand up to the politicians and media.
He has gone after the establishment politicians since day one, they know their gravy train is about to be derailed.
He is their worst nightmare, someone who actually has principles...
I would say that about sums it up. Trump is about to end their little fiefdoms, and they don’t like it. Screw them.