This wasn’t marketed like other superhero movies that expect huge opening weekends. It wasn’t really even marketed like a typical blockbuster (e.g., end of the world stakes, etc.) The filmmakers tried to be very clear about what kind of story audiences would be seeing.
It’s a great departure from what audiences have come to expect from the genre and it surpassed expectations, so I’m not sure I understand the complaint. Is it not enough to be a good movie that a lot of people really like?