Partly it's the art style of the series I've met, that sketchy Racish thin-point nib and lack of toner one. I'm a fan of ink, which I tend to find more in shoujo fantasy than BL proper. Partly it's the complete lack of turn-on in those two main BL tropes, schoolboys and salarymen. Partly it's the deadly sameness of run of the mill BL, which has been deadly same practically since its inception. Oh yes, I remember the Gust days; there was energy there to start, but in short order we descended into the Biblos B-Boy 'hire them straight from Comiket' manga and got artists who hadn't learned to tell a proper story or draw a distinct style.
And partly there's just so freaking much of it. The spirit faints within, especially when the spirit's eyesight is no hot hell either.
I'm a fantasy fan. What I like is stuff like Ze and Kohri no Mamono. The first is certainly BL, but I could happily dispense with the sex scenes. The second is debatable: the emotion is homoerotic but there's no sex. Which is fine by me, but then why is Kohri BL and not shoujo?
Even back in the day I was more a fan of series-derived yaoi than BL. The first dealt with characters I know and loved, and did things unimaginable with them. The second, very much running to one-off stories, dealt with people I had no reason to care about, and its only novelty was portraying sex. Sex gets old fast, or does for me. And there's the ephemeral nature of manga itself. It's designed for a specific age group that the artists and publishers both assume will grow out of it. The best of it may appeal to a wider and older audience-- Ima Ichiko, Mushishi, the best of the 49ers-- but the bulk is meant for fifteen year olds, or twelve year olds, and it shows.