What if we get some movies set in the past and others in the present?
That could be too much for the mainstream viewers. Following a set of movies with just 1 cast is easier for people who aren't big fans of the X-Men, well it worked for the original trilogy and they got more money with each new film.
"This is our chance to make a difference" - Sue Storm - The Invisible Woman
"He's stronger than any of us but he's not stronger than all of us" - Reed Richards - Mr. Fantastic
"We could these powers to help people" - Johnny Storm - The Human Torch
"You can't fix this, nobody can" - Ben Grimm - The Thing