This is a case in which the value of the schools will be measured (for The B1G) in the academic reputation, historical celebrity, and (for both B1G and SEC) media marketability. The athletics achievements of these schools are not the prize being sought.
The SEC could most likely have added UNC and UVa by now, if they really wanted them. So why would they want to add them now? They don't; It's their media partner, ESPN, who would be wounded if The B1G added those schools. The B1G footprint would be intruding into the territory exclusively controlled by ESPN. The B1G media footprint is already the priciest TV real estate in the business. Adding these two states would cause the price of B1G media rights to soar. If the B1G were bold enough to add Georgia Tech and/or a Florida school, ESPN would suffer an even more severe blow to their ad rates. Right now, any national advertiser who wants the Saturday college football audience in the Southeast must do business with ESPN. So ESPN's ad rates are set accordingly high. If those advertisers could buy ads on B1G televised games and get coverage in the Southeast, as well, ESPN rates would need to be set at a more competitive level.
Once again, ESPN will try to move assets (college football programs) to a conference in which ESPN's interests are served, regardless of how it affects the other parties involved. If ESPN tells The SEC to gather up the ACC schools that might be targets of The B1G, The SEC will do it. They need to protect ESPN financially in order to protect their own media payout. I'm not fond of ESPN.