I find Laurie Mylroie's book on Bush vs. the Beltway to be convincingly argued.
What we have to understand is that the Agency was not always like that. Eight years of Clintoon larded the government with too many leftists.
Remember, the first thing Clintoon and Hitlary did was to fire ALL repeat ALL U.S. attorneys so they could stick their leftie cronies in those jobs.
He did the same thing with the CIA. He wanted Strobe Talbot (ye gods!) to take over the CIA!!!
GHWB headed the CIA at one point. I suspect that's why.