They aren't sports fans. One of the things I do most often on Reddit is participate in various gamethreads in /r/nfl, /r/cfb, /r/hockey, /r/baseball, etc etc etc. There are gamethreads on Lemmy, mostly on the fanaticus.social instance. I'm the only one I've seen post in them, except for bots.
They aren't my kind of nerd either. All the communities for my hobbies are dead. The largest Magic: The Gathering communities go days without posts, the warhammer communities get maybe 1 paintjob post a week, and I haven't found anywhere to talk about how awesome the X-Men comics are right now.
At least you've gotten a post from the community in the past week, it's been months since anybody posted anything in only battlebots community on lemmy. I know it's the off-season, but still...