Every now and again, it's useful to remind ourselves of a couple of facts about the history of our hobby:
First, Dungeons & Dragons, by virtue of its being the foundational game of our hobby, has always commanded the largest number of roleplayers. This remains true even today. No other tabletop RPG has ever come close to the popularity of D&D at its height.
Second, Dungeons & Dragons, by virtue of its age, has a significant advantage over even its oldest rivals in terms of making its name nearly synonymous with "roleplaying game" in the mind of many people, including gamers who know well there are plenty of other RPGs.
Third, Dungeons & Dragons, by virtue of its being fantasy, is extremely open-ended in nature, making it a lot easier to accommodate a wide variety of not just content but also styles of play.
Given this, is it really any wonder that D&D has such pride of place in the old school gaming world? I'm regularly baffled by the periodic kvetching about how old school gamers are only interested in D&D, because I think it can fairly truthfully be said that the interest of old schoolers in D&D is no greater than that of most gamers in D&D. Indeed, I'd even venture to say that old schoolers, particularly those actively involved in this amorphous thing we call the old school renaissance, are probably more likely to talk about and play lesser known RPGs from the past than are most gamers.
Even a cursory examination of old school blogs and forums would show that their denizens evince far more genuine interest in hoary games like Boot Hill, Gamma World, Stormbringer, and Empire of the Petal Throne (to cite just a few examples off the top of my head) than does the general gaming population. Does such interest come close to rivaling that in D&D? Not by a long such, but, again, I would argue forcefully that's that true of the hobby as a whole and always has been.
Dungeons & Dragons and its derivatives was, is, and probably always shall be the 800-lb. gorilla of our hobby. D&D was the first published RPG ever and has had nearly 40 years to ensconce itself as the king of the hill. Even taking into account the game's rocky history, particularly in the '90s, no other RPG has ever displaced it in this role -- however much some might wish it were otherwise.