Re: 'X-men: DOFP' Official CAST Thread (announcements and discussion)
Well all of them were introduced before in the movies and none of them died in X3. I don't think it has something to do with staying true to the line-up of the comic version.
With Colossus coming back, the roster looks like what would FOX do if X4 was made after X3. Besides the 4 new cast members, all of the characters confirmed right now appeared in X3.