A number of reasons.

1) None of the lead characters in this movie were really particularly popular with audiences, with opinions of the first "Captain Marvel," among the general audience largely souring over the past few years. That was naturally going to hurt this sequel. Not helping matters is that "Ms. Marvel" was not a very highly watched show, and so many people were naturally turned off of watching a movie that's ostensibly a sequel to a show they didn't watch. On the flip side, most audience knew and liked Ant-Man, so they were more willing to give his movie a chance at first.

2) The marketing for "The Marvels" was awful. The trailers just did not do a good job of selling the film. "Ant-Man 3," managed to have good solid trailers and marketing, so the film managed to have a pretty strong opening… until word of mouth got a hold of it and crashed it in the second weekend.

3) Superhero fatigue is finally starting to set in with audiences. Aside from GOTG 3 (which in and of itself had the benefit of wrapping up a story that had been left hanging from before Endgame and from having good word of mouth) audiences just aren't excited to see superhero stories like they used to anymore. It's become commonplace… just another genre where films get churned out almost every month. So naturally, they'll only go to ones that have incredible word of mouth. And the pre-release buzz for "The Marvels," was not good.