I am pansexual/bisexual so I can’t speak as a “het” viewer but I can speak as someone who finds men attractive.
1) two hot men being hot together is hotter than one (lol)
2) BL stories often feature men behaving in ways that aren’t common in real life, such as being particularly emotionally sensitive or vulnerable. these kind of characters appeal to me.
3) I agree with what was said above about not having to insert myself into the story because there are no women in it. Stories with pretty women make me feel pressure to be pretty and behave in certain ways. If there are no women I can just enjoy the romance and NC scenes without thinking about what i should or would do.
4) i Like and relate in a positive way to queer stories. I don’t mind thinking about myself in the story when it’s about coming out or homophobia. I have been through these experiences and find it moving to see others go through them.
5) most mainstream beautiful women don’t suit my taste in women. I like butch women and powerful women and non binary people. Het or GL stories rarely show these types. I like pretty men and BLs show lots of them.
6) being queer and behaving in ways that go against societal norms is appealing and sexy. So queer men are sexier to me than straight men, on screen and in real life. (This is probably why i married someone nonbinary, born female. The men I like aren’t into me.)
there are lots of videos online about why straight women like BLs. Here’s one:
https://youtu.be/BrDML1ZaNaM?si=TeG6W7RNMBuHR7s3Y