I wondered about that. When X-Men Origins: Wolverine came out, the way it was titled made it seem like they were going to do a bunch of prequels exploring the origins of each of the main characters. Then X-Men: First Class came out and I thought "Okay, I guess not."
But a little research reveals that this movie was originally going to be X-Men Origins: Magneto as you pointed out. Maybe they realized that it's going to take like ten years or more to get through them all, and people will be totally sick of X-Men by then.
