There aren't as many forums for bass players as for guitar players, because there aren't as many bassists. And since the basic mechanics of pickups (electronics generally) are the same for guitars and basses -- the only real differences involve working frequencies -- most of the information shared in guitar forums is just as useful for bass players. When I search the web about some kind of bass-relevant electronics topic, most of the time it's answered to my satisfaction in a guitar forum or blog post, and that includes somewhat esoteric issues like Q filters and varitone switches.
I think the other part of the matter is that bassists and guitarists often have different goals in mind. Many guitarists obsessed with pickups are trying to recreate very specific historical sounds, to the extent of being kind of hypochondriac about the subtleties in the sound. There are plenty of bassists who are also into historical recreation but most just want to be heard, and their interest in tone is mostly about pickups that sound pleasing to them while being sufficiently audible in a mix. And that's a much easier target than a constantly fleeting idea of exactly what Jimmy Page's guitar sounded like at some specific concert.