Pretty much all of what @punkarsenic said. BL is mostly just a fantasy for straight teen/young adult girls. They are akin to those cheesy romance novels middle-aged women gobble up.
I'll add on my own observations, because I do enjoy seeing mlm relationships (hell, most of my series have at least one mlm ship). But I hate most of the BL that I see here and other sites.
Why I like it: I like looking at men, so it's double the fun if there's two in a couple. I prefer seeing men aroused as opposed to women I guess. It's more apparent. And I like that they can be vulnerable despite a socially or personally constructed rough exterior, giving into something more carnal.
The heavy Japanese influence - So yeah Japan made yaoi/bl suuuuper popular in the West and many people were inspired to create their own stories. The problem is that many of these stories fall into the same tropes. You all know the tropes, just google them. They become very obvious when you've read enough.
But tropes aside, the eastern influence has also shaped other things.
Writing - I'm sick of all the "kyaaas", "eh?s", and corny ass lines or people apologizing for silly little things and stupid misunderstandings. Especially if the story is set outside of Japan/Asia. Yeah, Japan has a very proper culture focusing on politeness and such but not in the damn west.
Art - So Japanese men are much less hairy compared to other ethnic backgrounds and this has translated into how their characters are represented. Everyone’s perfectly good-looking and I just find it so odd when in BL stories the men aren't Asian and they don't have any body hair at all. And it's funny when I see yaoi fangirls getting grossed out by hair. It's like, honey, if you aren't planning on dating a guy that shaves his body every week, you've got another thing coming.
Age range – many of these BL focus on characters who are barely 18 (you know, just to get by the age minimum so you ain’t creating kiddie porn). And while that has a place for young adults discovering themselves, I’ve seen far too much. I want to read about older characters. Look at real life, there are plenty of cute older couples.
Gay culture – Even though this doesn’t always have to be a focus, a lot of BL I see don’t seem to be aware of it in general. I know it varies by region and such, I’d just like every one who’s made/making a BL to watch at least one documentary/movie or read one book by a gay man. (I really like the docu “Do I Sound Gay?” on Netflix) Also, are the creators aware of what gay men are actually into? Yes, yes, I know everybody’s different, but I don’t see nearly enough bear appreciation. Gosh, I’ve come across so many bear blogs on tumblr. I’ll admit I like to browse them just to get a sense of what the blogger likes.
Tone – Seems a lot of these stories are either depressing as hell or diabetes-inducingly sweet. This goes for romance in general. Romance has its ups and downs, I’d like to read something “realistic” and natural.
Why the general hate?
Because regardless of how bad the writing or art is, slapping a BL on the thumbnail will get subscribers.