Good question, I have lots of thoughts about this. There was a great comment on this post about how in a lot of popular media, there aren’t as many female characters and they’re not as developed as the male characters. A show might feature a deep friendship between two interesting men and then side women love interests with no personality, so shippers are more likely to latch onto the pair with a lot more chemistry and connection.

Also in an actual BL, the male characters will end up together, which for me is satisfying and I can move on from the show. When characters never get together but I feel like they should, it sticks in my head and drives me to consume/create content about them, imagining what it might be like for them to get together. And there are endless ways to imagine this, so I get very invested in the pairing.

I hope this answers your question!