Looks like legendary X-Men writer Chris Claremont blames Hollywood for the X-Men titles dropping in sales. Because of Fox owning the film rights to X-Men, Marvel is not promoting the comics as much because they want to focus on the Marvel Cinematic Universe and the titles that Marvel/Disney owns the film rights to. They've even told their writers to not create any new X-Men characters.

X-Men Comics Writer Chris Claremont Blames Hollywood | Collider