[dimmke]

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.

[OJFord]

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.