It's popular because Hachiman is awesome and gets shit done despite being insanely anti-social and cynical.

It is also terribly realistic at times (at least from the perspective of RL loners), especially when portraying human relationships. I don't think I've ever empathized with a harem (it's not really a harem) protagonist more.

I agree that it isn't for everyone. I'd actually wager that a good chunk of people who like it just enjoy the cute character designs.