The X-Men franchise had quite a rocky road under 20th Century Fox, one which still, despite Disney acquiring the studio, still has not reached its end. The final Fox released X-Men film, Dark Phoenix, was a critical and commercial flop, and the final film produced by the studio, The New Mutants, still hasn’t seen the light of day.
Article source: https://www.cinemablend.com/news/2492894/why-the-new-mutants-director-felt-less-pressure-after-dark-phoenix-flopped