Male homoeroticism has been a prominent feature in fiction for basically as long as fiction has existed.
True, but we know 100% sure in most cases that the guys are indeed straight without doubt, while in yuri they go in that maybe she's gay, maybe not, you never know, cause they act super gay but call themselves friends, never question their sexuality, and even thought they're obviously attracted to each other they'll never ever think of the possibility of being gay even if they see each other naked, touch each other, grope, kiss and so on, at the end they will go closest possible to crossing the line but we'll end with something like "I hope we can always be together/I love you, my best friend~~", you don't see that with guys, never...
I can probably answer that, at least in an extremely broad an general sense.
Females tend to place more importance on romantic elements while males tend to place more importance on non-romance (humor, cuteness, sexiness, etc). This is why female-targeted romance series the characters tend to get together near immediately, and the relationship is always progressing in some tangible way, while male-targeted romance series will either have no progression of any kind or, when trying to be serious, the characters overdramatize fairly simple issues and agonize over them ad nauseam, never actually resolving them. It is also possibly why a number of people here refuse to accept and yuri series without a real, serious, definite relationship.
Anyway, BL as a genre is basically exclusively read by females, so it generally needs to have heavy and more serious romantic elements. Basically no males read it. And for the girls that just want to watch hot guys be hot, there's shounen sports to satisfy them. So there's not a huge demand for stuff that is more subtle and in the middle. The nearest thing you could probably call a middle ground is reverse harem, where the guys will often act somewhat homoerotic toward each other, but are still obviously dedicated entirely to the main heroine.
Meanwhile, the demographic that reads yuri is a more mixed bag. You've got lesbian girls that want to see real romance between two women. You've got men who care more about the girls and situations being cute / sexy than they do about any romance. You've got straight girls who just like cute things, and just want to watch girls look and act cute, so they don't care as much about serious romance.
So yuri is more fractured than BL because the demographic that reads it is much more varied.