A thousand factors contributed to Dark Phoenix bombing:
-Director: Not a big name, inexperienced, the shaky parts of his resume (Writing Last Stand) won out against his supposedly good rep with studio/actors
-Cast: Fassbender & McAvoy are decently liked but not to Hugh Jackman levels, only thing people say about JLaw were that she seemed to not want to be there
-Rehash of one of the worst x-men movies
-Last main outing, Apocalypse, was a tremendous disappointment
-Disney acquisition: journalists and enthusiasts who get the word out knew that this franchise was coming to an unceremonious end NOT on their own terms
-Bad word of mouth in general from screenings and things, news reports of reshoots etc
-Boring trailers
-Bad reviews when it did actually come out
-Probably a bleed effect from the continued failing saga of New Mutants contributing to a general 'they don't know what the fuck they're doing'
-Chastain, their big new hire, was not even a major comics character
Probably a bunch of other things too, it really was a perfect storm.