I think a huge part of the reason was the misleading marketing. Example:
“The X-Men face their greatest foe...their own.”
“Everyone has a dark side”
When...
spoilers:end of spoilers
Jean is not the villain or truly turns to the dark side. The film is rather about her evolving beyond that dark path she had in X3.
I having seen MANY people complain about the lack of Jean going all out.