The thing is that most users of reddit and discord don't care about older topics. Go to any large sub with lax moderation and watch how many times the same questions are posted.

For many communities, it doesn't matter if its a reddit replacement with the reddit functionality but rather that it is a place to host that community (that will adapt to and complain about the confines of their new home).

It only lacks real functionality as a reddit replacement for those people who are lurkers and want to consume a digest of past content (and maybe add a forgettable comment to the end of a too long comment tree).