I think it's just more delicate than CU because of that and some other things, not a problem any more than it was a problem for OU to leave OKState behind.
I've been talking to someone connected with AZ and it seems the vast majority of influential forces there are strongly for B12 and have been for quite a while. I think with ASU and Utah the rivalry has gotten in their way to their own detriment. It's not unlike when people force themselves onto opposite sides politically by reflex, sometimes everybody should be on the same side of an issue even if it means getting cozy with rivals.
AZ loves the idea of getting into an elite basketball conference and they lost the one elite hoops conference rival they had. There's nothing holding them back.
If Arizona and ASU share a BOR that's an issue as well. Even if CU and CSU share a BOR CU jumping to the Big 12 doesn't affect CSU in any way. But it would be an issue for Arizona because it would mean separating AZ from ASU... and ASU would likely be left in a rough spot as a result, although it's one of their own making due to their attitude over the last several years. 2 years ago the Big 12 would have easily taken both, but with ASU's attitude they're really not a wanted entity anymore.