The differences between the ACC financial distribution to its member institutions and what the SEC and BIG distributes to their members is so significant, that alone could explain their motivation. For FSU, its also a weak excuse to blame the ACC for the manner in which they were dismissed so quickly from the National Championship consideration.
While I am not sure this is a fair argument, it is the one argument that has long been popular inside the SEC, BIG and with the National Media .............. specifically, if FSU or Clemson had to play the SEC football schedule, neither one would likely qualify for a Conference Championship. I am not sure that same argument would apply as much to the BIG, but clearly the ACC has done little to raise its own performance level.
One last thought ..... it is not beyond the realm of possibility that ESPN would encourage either the SEC or the BIG to offer one or both (FSU & Clemson) into those two expanding Power Conferences as a way to force the ACC to merge with the B12 remnant, and reducing the required TV revenue they need to pay out.