For so many, many reasons.
First off, most of First Class is dead, right?
Second off, Hugh Jackman is a fan favorite. If they do a movie set in the 80s or whatever, he will have no memory of DOFP (since his memories only return in 2014 or whatever).
Third off, that's a lot of movies that have been set in the past, at this point. And while the older generation loves the little historical nods, they're probably starting to run out of references, unless Wolverine is going to pull a Forrest Gump.
Fourth, I don't think they would have made such a big deal out of the fact that Jean and Cyclops were alive again, only to completely ignore them.