There is something called a scope clause, and almost every major airline has one except for Alaska.

United's scope clause says that the regional partners can fly:
50 seaters, total numbers up to 90% the number of single aisle aircraft in the fleet
70 seaters: up to 255 (combined 70/76 seaters)
76 seaters: up to 153


Since UA will max out the 76 seaters at 153, they can max out the 70 seaters at 102 (153 plus 102 = 255)

Between GoJet, Mesa, and SkyWest, there are 77 CRJ700s being operated for United. Republic operates 38 E170s. Once all the E175 deliveries have been made for Mesa, Republic, and SkyWest (153 combined), 13 CRJ700s will have to leave. There will still be a need for CRJ700s in the near future for certain airports (MMH, ASE, SUN, JAC, EGE, etc).


Also, most major airlines do not like to give all of their regional flying to just one regional partner. This is akin to putting all of your eggs in one basket. Imagine if all United Express flying was done by one regional, lets make up a name and call them United Express. Imagine that the pilots of this airline decide to go on strike. Now, United doesn't have any regional feed. If you spread it out between SkyWest, Mesa, GoJet, Republic, Air Wisconsin, Expressjet, and Commutair, and one of them goes on strike then its not so bad. Also, the different airlines can bid against each other to win the contract. Lowest bidder wins, keeping costs low.