hey friend
I can’t give you any real specifics because I don’t have a dick and I’m also asexual, but I think it’s that not every person with a cock can maintain an erection while getting anally played with even if they enjoy it very much.
I also guess since like… 80% of an orgasm is just in the head, those persons don’t actually need their cock getting hard since they get all their satisfaction out of those anal orgasms?
I can’t really tell you… it’s just what I think lol