Because he was a hot dogging cowboy asshat that thought he was flying a fighter. The revolt of any crew to fly with him should have been a huge eye opener. Personally if he'd called me a ***** after protesting about clearing a ridgeline at 3' I'd probably beat him senseless and let the chips fall from there.

"Why it happened cannot be determined..."

Agreed. But he was also in charge of the Wing's Standardization and Evaluation branch, which means that somebody up the chain-of-command was favorably impressed with his flying, and had charged him with impressing upon those in his organization his distorted sense of values and standards. That opens up a whole new chain of responsibility for the accident.

I think the OP was asking about the "why" from an aerodynamic perspective. How much altitude does it take to recover a B-52 from a 90 degree bank at minimum airspeed? How much from a stall in that condition? I doubt many B-52 pilots know, but I would guess at least 10,000' would be as good a place as any to start.

Big airplanes are not fighters. Sometimes its better just to park them on the ramp and let the airplane speak for itself.

Nepotism...

He should have lost his wings after almost hit a ridge line, trying to show off for the camera!

Nepotism? Was he related to a someone up the chain?