They never said.
there were rumors about shoping HB as a tv show, but that was proved sadly proved false.
Im going to guess that the dvds didn't sell well, and Stars didn't feel like green lighting a 3rd. If Hellboy 2 does well it wouldn't surprise me if Universal made them start on another cartoon.
They sold well, but not well enough. I think Starz was expecting more, without actually putting forth the effort to promote the films and make an awareness of out there.
HB animated was initially supposed to be a TV show, but they decided to do it as movies instead. Del Toro thinks there's still a chance of reviving the Hellboy in animated form after the live action sequel is out. If it does well enough, that is.