Makes sense: the two Hellboy films only became profitable with home video in the mix, and now that the home video market is smaller studios don't see HB3 as a safe investment.
... still, I love the films we got, and HB2 ends with the heroes in a nice place. The comics are still incredible, and coming out at quite a clip. There's so much cool stuff in the comics that the movies didn't even begin to touch. I can see a (gasp) rebooted Hellboy film franchise in, let's say, a few decades time. Maybe a TV show? TV's already on its way to being as big as film (I'm including direct-to-the internet shows, too).