The most obvious issue for me is that despite all of the social changes that have taken place over the last century, for the most part, we generally still see pregnancy as something that women do (or as something that happens to women). Of course, it is something that takes place within women’s bodies. But even the increasing number of people who say things like “we’re pregnant” doesn’t change the fact that for most people, it’s “she’s pregnant.” And that’s reflected in the way that men talk about hormonal birth control.
There’s a lot of resistance from guys around taking a birth control pill that has unwanted effects, even though women have been doing that for decades and that the male versions seem to have fewer unwanted effects. And a lot of men are caught up in the idea that fertility = masculinity, even though women have been able to overcome their version of that. I think that part of the issue is that there’s nobody championing male birth control and modeling that taking a birth control pill doesn’t make you less of a man.
Of course, a male pill wouldn’t protect you against STI. And there’s always the question of how you’d know for sure that your partner was using it. But if you have the sorts of sex that make birth contol relevant, what would keep you (or your partner) from using it? What would make it more likely?