It's mostly because there are different sites for the community to be in contact, be it discord, the steam group and the IRC chat. When the forums started those weren't pretty common, so all the conversation, showcasing and fooling around happened here. Now the art thread has no replies because we have the #creative channel in discord, same for sprites, the avatar thread isn't really relevant because people don't change their avatars here too often.
This add to the fact that people aren't doing more mods or mappacks, or characters or hats. And we haven't done any forum event, or made a thread for a recent topic (like in the mfgg forums).
We could make a mappack contest, or sprite/drawing/tiles contest, that may increase the activity a bit, guess.