To be fair, "Hellboy" was the first big budget foray into an independant comic production for cinema. Not a very well know comic book, but excellent character base. Off the back of that came "Sin City"(from the same label) and "Constantine"(originally a Brit, by the way!) from Hellblazer comics, if I remember as my bro read them.
They took a chance with Hellboy and the budget wasn't huge because of this. It generated moderate success and a sequel was always likely as a result, going bigger and better storylines, with more confidence. That said, I doubt you'll see more than 3 films.
For what it's worth, I really liked the film. It was different from any other comic adaptation and could see what they wanted to do with it.
