I think a lot of people also have superhero movie fatigue. After all those Marvel movies, DC movies and whatever else since Iron Man 1 basically, a lot of people are fatigued enough by it to not actually go to the movies to see it. You have a certain amount of people that will always go see all of that like me lol but as far as the average moviegoer that doesn't go to the movies much anyway PLUS probably doesn't know much about comic books or care about them, they don't want to see it in the theater and that makes it hard to make a ton of money on a superhero movie.
You used to could count on a Superman/Batman/Spiderman movie to make a lot of $$$ no matter what but now it's just not a sure thing. Spidey movies still do well so far but it just seems like it's trended that way to me. It's a shame too because when there is a good movie like I think Supergirl was, it doesn't matter because it didn't make enough money to recoup it's costs. Word of mouth could maybe help after people see it but it probably won't change enough minds to make a big difference. And more and more people have the "I'll just watch it at home when it streams" mentality. I don't know if those reasons are why some of these movies are failing in the box office even when they seem to be a good film but that's my opinion. Hope it changes in the future.
I hate rude behavior in a man. Won't tolerate it. - Woodrow F. Call