Well, using MMORPGs and The Guild as a comparison point, we can break this down into a rather rough hew analysis.
1) Is there a central property? Not, of course, one game to rule them all, but a hegemon. TT RPGs have D&D. MMORPGs have WoW. At the time that The Guild came out, WoW was the MMORPG, in a way that D&D hasn't been since the 80s heyday. Even if D&D (+ Pathfinder) is the biggest share of RPG-being-bought, there isn't a single edition that everyone plays and everyone will agree as a good referent.
2) Is there a central locus? I assume that there's a main forum/board/other main site that is a clearinghouse for WoW culturally tangential material where someone might link to The Guild and lots of people would check out. What's that for TTRPGs? Enworld? Dragonsfoot? TheRPGsite? RPG.net? Certainly not WotC. And that's just the RPG players who are surfing for RPG related material, which brings me to...
3) Is there an online presence? Yes on both counts, but almost by definition, all MMORPG players are online. Not that anyone is offline these days, but I'll hazard a guess that a majority of RPG players are not online looking for more RPG related stuff. I mean this board is a pretty big one, and how much traffic does it see in a given day?
This is all completely unscientific, and my epidemiology or study design profs would kick me for the lack of rigor, but those seem to be good prima facie explanations to me.