Fair enough, though not exactly the answer.
sigh Ok, so there are exceptions to all rules but in general, the male brain is programmed for the visual. For the imagery and the sounds and to hell with the story. That's why magazines and videos are marketed to them far more than women.
Women in general find story more pleasing. They need to know more about the characters and want to fantasize about them and dream up things in their head. This is why they get marketed romance novels. A few are actually really interesting. But for some, crap like 50 shades is good enough. There are womanxwoman romance novels as well.
BL, Yaoi, Gay Romance, and even anime like Free! are marketed to a female audience.
Now some of it really is porn. It really really is. It may be artistic and pretty, or well worded, but that's just what it is. On the other hand, some of it also has long sweeping story-lines at the front as the star of the story with the gay premise being only secondary or tertiary.
I think part of the issue is that the original question is still fairly broad. Or maybe all the comments broadened it. Hmm... but in guessing that they meant 'BL' as 'gay romance of any kind' it really is a big blanket.
Also, it's popular in some places, not others. Really depends on the community and where you look.