Beyond the obvious, I think there's two reasons.

1) Booby was not a high level associate A.D. at SDSU, so he probably didn't develop a lot of professional contacts outside of SDSU. Most of his contacts are at SDSU, so he keeps going back to what he's comfortable.

2) He's probably risk averse or just damn lazy. Doing a "national search" and contacting people for background checks is too much work for him. He'd rather simply rely on contacting the only people he knows in the industry.

Maybe over time, he'll develop more close relationships with other A.D.'s which will be instrumental in future hires.

That said, I hope he got lucky and hired a good coach.