I'm not from reddit but I have thoughts nonetheless.
if you get attention to your thread via some ”easy” method like having the most recent reply, this attracts stealth abuse. troublemakers can try to use plausible replies to push up what they want to fill the top, and push down everything else. i want to emphasize that you cannot really notice or be sure when this goes on.
therefore, it's important that popular clients don't make anything like that the default. upvotes is much harder to game. recent post (but not replies) is okay because people making too many dumb posts is obvious and they can be blocked (etc.). most replies is fine since it takes lots of different people. even some combination (weighted upvotes, weighted downvotes, weighted total replies) could be nice.
instances could take how their ”front page” / default sort works very seriously because it ends up shaping what users see and interact with the most.