Florida State is bigger than any Big 12 brand. Clemson likely is as well. The Big 12 is the more secure league because they then probably have the next six-eight marketable brands after those two, but not so "marketable" as to be interesting enough to the SEC or B1G.
Miami is good at spending on the football, but their brand isn't as big as much of the Big 12. They're heard of, sure, but just don't bring the eyeballs.
UNC isn't good at the sport that matters for media dollars, and they don't seem to outwardly care about it. They're not a brand the B1G or SEC would have much interest in until they fix that (which is probably why the weird desperation play for Belichick).