Part of Trump's challenge is that potential nominees are seeing that Trump won't be loyal to them either.  A good example of this is with Rex Tillerson, where Trump has made major foreign policy blunders without even consulting his Secretary of State.  The most recent being the Israel two-state about-face.  Tillers heard it from aides when he landed. 

Who wants to take on a role like that when Trump will parachute in and undermine everything you're working on?  I'm sure lots of smart, qualified people are ducking being a nominee for that very reason.