Simply put, it's everywhere because there's a demand for it.
While overall LGBT content is gaining traction and building an audience, BL has the advantage of being around since the 2000's with a vast percentage of its audience being straight women.
Much of it is just a straight-up escapist fantasy with the 'uke' as a thinly-veiled stand-in for a woman, hence the big draw. And since a lot of these women aren't part of the LGBT community, there never really was a demand for BL to be more honest and truthful to the real lives of gay men.
Of course BL written by LGBT creators exist and are more sensitive to the community, but those are just a bucketful in an ocean of straight-made wish-fulfillment.
Personally I'm irked 'cause I'm like, "Melissa, if you want to explore your sexuality, just write a female character! Break that societal mold! Don't put it on poor old Mel, the waif of a brunette with green eyes and freckles who stands at 5'5" and blushes every time he hears the hot guy speak at the coffee shop he's working at!"
Heckin' own your sexuality as a woman! The world needs it.