The ACC is just more "new blood" than the PAC-12. Nearly half of the ACC has only been in the conference since 2004, and a big chunk of that are refugees from a conference that had to change its name 2 years ago. The PAC's membership list has been consistent since the '70s outside of the recent additions of defecting P5 school and arguably the hottest G5 school in the country at the time. The PAC has much more of a shared history and togetherness vibe than the ACC. Plus, they have about a third of the country pretty much all to themselves out West; things are pretty crowded on the East coast.
Look no further than these conversations we're talking about half a decade ago - the ACC conversation was "FSU and Clemson/Miam may leave, can the conference survive?" and the PAC conversation was "are they going to destroy another P5 conference and and pilfer the schools they want?"
In the Big XII's defense, no matter how unrealistic it was, if they weren't going to be hoping for schools like FSU, Clemson, and Louisville to defect the next level down is the ::cough:: "less than impressive" group they're working with now. With that as the alternative I think being a bit pie-in-the-sky is acceptable.