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.