Well, let me tell you a story: Once upon a time, there were a group of women who, with their various demon acts, broke men apart. Chipping away at their souls and minds, men with each generation males stayed indoors becoming weaker, eventually leading to the dominant rise of the Females ruling society, generally remembered as the rise of the Feminist Nazis All this changed when two men, Sub Uke and Dom Seme, realized the discovery of the magic called BL. By utilizing this magic, the two men found that they now had the power to not only fight against the tyranny Female Nazis, they could also end it. The two men then spread out the knowledge and then created the brotherhood and organization: Yaoi. With both the powers and numbers together, the brotherhood then set out to eliminate their overlords. With victories and losses, thought Yaoi has won the battle of those who oppressed them, they have not won the overall war. Even with the Femi-nazis decreasing from the Yaoi rebellion and the civil war with the female organization, Lily, both The Yaoi and eventually made a non-aggressive pact. And so, with the decades and eons passing, even with the people who have now forgot the wars that marked history, the Yaoi still remain within the shadows, quietly reminding the magic which lies in BL.
So whenever you read a yaoi, remember, whenever you get confused or "the other you" reacts at something different, know that you are awakening to the magnificent magic powers of BL.
Jokes aside, it mostly has to do with the culture in Japan. There's a set standard and generalized genre which male mangakas had, then some female mangakas came along and they were like "what should I be drawing to garner readers?" then boom, you get Dom Seme and friggin Sub Uke. I srsly dont know what happened with the history behind this. Anyways, Fujoshis and most female mangakas, because they suck at every other genre and were more interested in making guys be pinned down and dominated by other guys, gave the result of over-production of BL. Yuri on the other hand isn't as popular due to the majority who work on them, are in the minority of interest. The fandom on the other hand, is only interested in yugivmefeelings yuri or animated Yuri that has bear girls licking honey covered lilys covering over another girl and some other bear girl licking along side that first bear girl and the video playing in slow motion with--- I digress but yeah that should answer a lot for yah.