Whoever said contracts is right. In Football, the video game developer makes an agreement with the Players union and the team owners and they get all the players.

Boxing has different promoters, managers, and is very splintered. I think they have to negotiate with each fighter individually and pay them royalties to make it happen. Probably alot of fighters that aren't popular now are priced out and the developer doesn't want to pay for something that isn't going to appeal to the masses.