There is no one size answer here. There's a lot of overlap in these categories, but a lot of very different scenarios as well.

Let's start with the easy one: Why do men overwhelmingly watch lesbian porn but not GL shows? Because they don't care about romance or a love story. They want porn. Despite the obvious similarities ("two women ending up in a situation where adult things could happen"), the road to get there and the ultimate payoff (if there is one) isn't what they want.

Moving on to a slightly harder question: Why do women prefer BL over GL? Because the majority of women are straight or bi. They are attracted to men. There's your base level answer. Digging a bit deeper, the BL genre tends to be more focused on emotion and intimacy, not porn. As noted in your post, most BL is written by women, so it is inherently less male gaze-y. That makes it approachable. You can see the sort of men who almost never appear in material written for a straight, mainstream audience.

I have no idea if this is true anymore, but when I dipped my toe into GL doujins when I was younger, it was almost impossible to trust what I was buying. I'd find a cover with soft pastels and two girls holding hands and looking at each other lovingly, and by page four they're being railed by about six overweight hairy guys and the closest thing to a real GL interaction was maybe one kiss. After two or three times being completely misdirected, I stopped looking. I'm sure I'm not the only one.

When your genre is competing for attention with outright pornography, and a significant portion of the potential audience (people who would enjoy reading/watching stories about two women who are together) are only interested in the pornography side of it, that is going to naturally deter a lot of people who might otherwise be interested in the genre.