This question is mostly concerned with live-action theatrical films.
As we can see, since around 2002 the number of films released per year grows significantly, mostly because of Marvel, though DC Comics releases films more often than before too.
Why? Notably many of the characters are decades old in comics. Did something prevent making so many films before the Millennium? Or did the general popularity of superhero-themed media (including comic books and animated series) grow? Do the movies target teenagers like animated series of 1980-1990s or those who were teenagers then?
Maybe there’s some publicly available market analysis discussing this.