It is to give the departee conference the chance to replace the departing school. It is also to get schools and conferences to get their spit together so they announce in the winter or spring rather than the summer. That is why it is 27 months instead of 24. So if the Big 12 announces it is adding UConn at the end of August, that means that realistically, UConn would not join until 2027. Given a likely "earn-in" into the new league, UConn would probably not get a full share of their Big 12 TV revenue until 2030 or so, which (not so) coincidentally is right around the end of the Big 12's current ESPN contract.
In other words, if UConn left the Big East, it would be taking a lot of risk on a new Big 12 linear contract that might end up being a lot less than the current one.