They're gonna release Dark Phoenix and New Mutants apparently, so it's not like they won't have anything out soon. I wonder if Marvel TV (i.e. Agents of Shield) might introduce some elements. The things is, everyone knows what a mutant is and their story like they know that Bruce Wayne's parents were shot. They need time to disassociate themselves from two movies that are gonna come out.

It would be useful for them to appear alongside the Avengers more and not a separate franchise as they've done throughout their media history.