Civilian control over the military. It is one of the foundational principles of our Republic. It is exercised through the President as commander in chief, and his primary national defense cabinet member, the SECDEF. A General is always a general, even after he retires. His is what got him there and cannot change that about himself. Thus, he carries with him all the institutional prejudices, biases and blind spots that molded him since early in his career. He drank the cool aid a long time ago and made a career out of serving the Kool aid to others.
Thus, he is unfit to be the master of the other Kool aid drinkers.
This is not a criticism of him. ALL GENERALS are procurers, mixers, and dispensers of Kool aid in all it’s colors and flavors. It is how they are made.
The DoD needs a new chief who is strong and capable and can tell the Generals to piss off. Mattis would never do that, because he is one of them.
ADDENDUM: Obviously, if Mattis is selected as SECDEF, I will definitely support him because that’s what I get paid for.