Good question! Looking at the years pre-Iron Man (which is obviously not precisely where really superheroes took over) on Box Office Mojo, the big films were Pirates Of The Caribbean, Harry Potter, Transformers, Mission Impossible etc, so there isn't a strong theme apart from a bit of "me-too" post-Lord-Of-The-Rings fantasy. However, looking at the last few years, there's only like four superhero movies in the Top 10 maximum*, so their dominance seems pretty overstated.
(*2018 looks to be the high watermark of superhero box office dominance, with Infinity War, Black Panther, Incredibles 2, Aquaman, Venom and Deadpool 2 all in the top 10.)