The ATC system in the USA is based on first come, first served basis. I believe we will see our larger airports implement a lottery for slots sometime in the near future.
The Regional Jets are used like pawns in a Chess game. For instance if a terminal gate isn't being used a certain number of times a day (KEWR) the airline who leases the gate must relinquish it for another carrier to use. Consequently, the gate lease holder will put the small jets on it at non peak hours.