Reddit has a standardized layout, you don't have to learn a new interface. It has single sign on, so you don't have to create a new account to participate in a community (low friction) and it has the people. Also, it has the voting concept which helps filter content so you aren't having to do that yourself by scanning topics.
A "classic forum" trying to get off the ground would be competing with all of that. You could also argue, "why Discord? Why not IRC?" for much the same reasons.
Yes, that is exactly what I suggested in comment previous to (up-thread of) the one you replied to.
As I said there, there is also Discourse, with only slightly more multi-community friction really, making it far easier and more standardised to do initial sign-up.
A "classic forum" trying to get off the ground would be competing with all of that. You could also argue, "why Discord? Why not IRC?" for much the same reasons.