Do you think UF will help FSU get into SEC or South Carolina will help Clemson? After the SEC expanded, i m sure there were all kinds of side deals getting cut. In those dealings i m willing to bet than alliances were formed to keep certain universities out of the SEC at any cost.
"Gentleman's Agreement" is what prevented GT from rejoining the SEC. Florida would never allow FSU to encroach on its SEC market in it's own backyard.
The exceptions of course are built-in historical rivalries which are viewed as an asset as opposed to a liability. Examples: Duke-UNC, FSU-Miami, BC-'Cuse, VT-UVA