I think part of it is that meme discussions are just a great place to actually talk shop about D&D.
Back on reddit, the vast majority of D&D subs were flooded with fan art and very little discussion. There were DM subs, but those obviously left out players. I loved /r/rpg, but that place was also a refuge for people who don’t want to talk about D&D and only D&D all the dang time. (And even if I’m mostly over D&D, I still like D&D in theory, if not always in practice). So… that kind of left /r/dndmemes as, unexpectedly, one of the few places to get in-depth discussion about all kinds of RPGs and experiences from around the table.
Somewhat off topic, but the names of both the publisher and the developer are also used by unrelated tabletop game companies. Hero Games makes the Hero System tabletop RPG, and GameScience (no space, so there’s a difference) make dice. It threw me for a loop.