There are 5 KINGSVILLE MOA's that are scheduled by CNATRA at Corpus Christi. I'm sure the Kingsville based T-45's have the highest priority of use there, but technically any military unit can ask to be scheduled there as time permits.
The T-45 is the last stop in the training command before graduating and moving on to the "real" fighters of the USN & USMC. The students first shot at landing on a carrier deck comes while flying the T-45. Lots of fun scary stuff to be done in those training units and they need lots of airspace to do it in.