I think it's a combination of the vicious cycle darkenergy mentioned and the scarcity blankd described. How does that work? Simple - there's not that much BL if you take webcomics (and especially all entertainment media) as a whole, it's all just concentrated here, in pockets on dA, and on y!gal. There's not a lot of it elsewhere, so fans flock to it where they can find it. The scarcity* means that even the crud 90% can get an audience.
Most BL authors make BL not for the fans exclusively, but because they want to make BL. Fans mainly determine whether they make a comic at all and how long they keep going. I think most BL authors would have stuck to BL doodles and illustrations if they didn't think they could get a bigger audience by doing a comic.
Although I'm sure there have been cases where someone not interested in BL made a BL comic just for the fans, I believe there are rare exceptions.
I could be imagining things, but I think BL's been declining in popularity on SmackJeeves since Admin kicked out some of the more popular graphic comics. I think the fangirls started to move away with the big comics.
* I think more than scarcity is at play when it comes to the fact that even a very low-quality BL comic can do pretty well. BL is associated with low quality and the fans mistake the status quo for the ideal. The first few works they read likely form their standard, their bar, but those works were very likely mediocre. They're simply not trained to see or care about flaws that people who got into comics through better-made comics find to be glaring problems. Many of them are young too, their lack of experience contributes to their low standards.