Because those schools aren't as attractive as USC, Oregon, UCLA, and Stanford. People think this is all about athletic success and to some extent it is true. Football is king. However, there's also academics that come into play. UNC, Duke, Virginia, etc aren't football blue bloods or even highly respected football programs. Don't get me wrong, they've had their moments in the sun in a weak ACC outside of Clemson and FSU. They aren't as attractive as the alternatives however. USC is still considered a blue blood.

Furthermore, getting back to the academics argument. The B1G isn't taking Clemson so you can take that option off the table. Why, you ask? They aren't AAU. AAU is supremely important to the B1G. It's why they've been on Nebraska's case to get it back and it's why there was a rumor that when Texas and Oklahoma expressed interest in the B1G they were turned down. Oklahoma isn't AAU. So it's safe to say if they're turning down a blue blood like Oklahoma then they're not going to take Clemson. Which leaves only the AAU schools in the ACC. As I've said, those AAU schools aren't as attractive as the Pac's AAU schools. Furthermore, it's been long rumored that Stanford would be added regardless of their athletic performance because it's Stanford and the B1G schools would love to be able to not only collaborate with the University itself but also have them in their conference as a sort of show pony.