Why is there a string of superheros movies in the past 10 years?

Why is it these days there are a lot of superhero movies in the
cinemas?

I think it started somewhere in the early 2000 with Spiderman. Then
there was the Hulk and subsequently a whole lot of superhero movies.
The latest being the Green Lantern.

It happened quite suddenly. If my memory serves me right, there was
the old Superman movies in the early 80s as well as the Batman movies
in the early 90s. But that is about all.

Lately, there are many superhero movies, one after the other. Before
the Green Lantern, I think there was Captain America and before that,
there was Thor.

I mean, they seem to come suddenly one after the other.

Why is it it seems a string of superhero movies are produced since
early 2000 to now?
Why did the directors start to produce superhero movies one after the
other in the last 10 years?
There were hardly any in the 90s except Batman if my memory serves me
right.
Oh yes, there was Iron Man. Good movie.