The reason to play zone is to counter specific offensive sets. LSU and Georgia come immediately to mind as teams that spread the floor, drew the bigs out of the paint, and were successful in getting dribble and backcut penetration to the basket. Zone would mitigate that to a large degree. It would also limit the ability of pick and roll offense to hurt you (obviously at the expense of giving up other offensive looks).
Our man to man is good enough that it usually won't matter, but it's not as simple as "man to man >> zone, end of story." I don't think M2M is by default better, though it is a better defense, all things considered, for our team to play.
The caveat for Cal and our team is the he has these guys for one, maybe two years for the most part. Hard to install a complex UL or Syracuse style zone defense and have them comfortable playing M2M and have them learn the offense in that shortened period of time. M2M with switches on all screens is absolutely the easiest concept to teach the players. Cal knows this.
FWIW, he did go zone for one possession against UGa. We looked lost, then gave up an offensive rebound and putback right in the middle of the paint. Bye, zone.