In the AvP Predator did gain a lot, and Alien had lost a lot.
The thing is, Predator franchise (the movies) has really limited expanded universe, all three movies are non connected, first movie has just one predator, second has several predators, predator ship, and scene showing trophy skulls of multiple species, and only the third one (that was shot after the AvP) starts to expand the things a bit more with multiple fractions, sub-species... it sets the grounds for a bit more deeper universe.
The Alien franchise on the other hand (again the movies) has a really deep expanded universe, with the company, the space marines, colonies.... etc.etc.
AvP really did had potential to enrich both franchises, the game Alien vs Predator 2 really worked, and it worked just great. And the best part of the game wasn't Predator or Alien campaign it was when playing from the colonial marine perspective.
If the movie wasn't about the Alien vs Predator, but about colonial marines trying to survive all that mess... that would have a potential to make a awesome movie.
However the movies have settings and characters that are simply there as an excuse for getting Aliens and Predators to brawl like idiots.
After that failed POS it's best to have Alien and Predator going their separate way.
Bunch of Predator fans feel butthurt for making AvP non-cannon, but it is for the best.