it was going to be a teenager horror movie under Fox but got some delays, then the Mouse bought Fox so that's another big delay and some reshoots (maybe to tone it down to be more family friendly, but also there's reports that Disney wasn't impressed i.e the movie was bad). They didn't know what to do with the movie but it got some god news this year that a new screening got more favorable results post the reshoots and a better cut
But I don't know, maybe Feige probably wants the synergy and even if it's a spin-off, it's still X-men.
People was doubting it would ever see the light but I still have hope. Hell, toss it on Hulu if anything.