I write BL because I'm lazy and cba with all the societal machinations needed to insert Strong Female Badass into story requiring two badasses. Having two dudes both be badasses is easier. Then when one turns full nuclear warlock badass and raises an undead army and imprisons his lover in a tower of thorns, no-one will call him a bitch and burn him like they would if a girl did it.
Joking aside, (although that pretty much sums it up on the surface), it's the same as others have said: equality in historical settings, two pretty dudes for price of one, no need to explain why/how one of the partners isn't getting a load of aggro from the populace about bullying an army into existence to help the other one, that sort of thing.
Even if you create a society that has equality, if one half of your couple is a woman, she will still be limited in ways the male isn't - physically. Ok so you can level them out by having her with magical powers and him with an army, but there might be a power imbalance the other way. If two men have same status, same abilities, then the power play comes more from their ideology rather than physical/status/gender limitations , I guess. Also as a dyed in the wool tomboy, I prefer masculine spirits. I think I write men better than women because of that.
I hate it when I see gay romances written by straight people just so they can indulge a fetish. It's pretty obvious, and they are out there. At the end of the day, I like a well written romance and really couldn't give a fuck who it's between, or if it's just one or several or whatever. As long as the emotional connection is the focus and not what their doing with their bits like. Although I'm done with bulky muscles, please stop, sometimes your hot hero can be a weed you know.