Could be he didn't have a choice. It's been rumoured that Brock is a master when it comes to negotiating and Vince was desperate to get him to do this year's Mania. The WWE issued the following announcement after the UFC announced Lesnar would fight at UFC 200.
Brock Lesnar remains under contract to WWE, however, he has been granted a one-off opportunity to compete at UFC 200. Following this milestone event on July 9, Brock will return to WWE for SummerSlam on Sunday, August 21, live on WWE Network.
Don't know what's in it for Vince and Co. but he's not a stupid man either. As for Rousey she would still draw, but just underwent surgery and she's still under contract to UFC. Besides since she lost it's been one excuse after another for her not to get back into the octagon. First she was injured, then filming a movie, now the surgery. No one knows for sure if she ever will make it back and what kind of shape she'll be in mentally and physically. Dana White has said she will get a title shot, but it remains to be seen when that will happen.