The reasons forums are better:
- threads are linear
You post something to a five year old discussion, everyone in the forum notices.
- no voting
Doesn't matter whether your opinion is controversial or not, it is presented as just as valid as all other opinions and thus is worth engaging with, fostering debate.
- smaller communities
It is actually possible to read every post made to the forum and know every member.
- niche interest as the main driver of everything
Everyone is aligned on at least something and politics don't pervade everything.
- moderation can still kill everything
If you got a mod willing to remove posts or ban users for differences of opinion (sexist, racist, homophobe, transphobe, islamophobe, etc. etc) then everything dies either way.