UNC and UVa are a slightly different situation in that they are the #1 programs in their respective markets. I do not see the Big 10 taking the #2 school in a market, no matter how big that market looks. You could make the case that Florida State is a lot more attractive than Georgia Tech because FSU has a higher profile and better market. The problem for the B1G is not just the school itself, but the schedule. FSU vs. Minnesota, Michigan State, Iowa, Indiana and Illinois is just not going to generate much excitement when Florida is playing Georgia, South Carolina and Tennessee on consecutive weekends.
And the BTN knows that even if it adds UVa and UNC, the SEC could add Virginia Tech and NC State, and while the BTN could compete on those markets, it would be a battle. Plus, the ACC will still be in the southern states no matter what happens, so there is even more competition in those markets. If, on the other hand, the BTN adds two northeastern schools, there is no competition.