Aside from the obvious fact of its being the 800 lb. gorilla of the hobby, one of the reasons that the term "old school" is almost always conjoined to "Dungeons & Dragons" is that D&D is one of only a handful of RPGs where there's a significant schism in its fanbase. There's clearly a "before" and an "after" in D&D, though precisely where that line is drawn can be a matter of debate. The point is that, when dealing with D&D, "old school" isn't just a synonym for "old;" it's a statement that, after a certain point, something changed and changed in a way some didn't find congenial.
Among the many, many roleplaying games produced since 1974, I suspect only a handful of them have changed enough over the years to make the use of the term "old school" in reference to them meaningful. The two most obvious are Traveller and RuneQuest, as I've noted before. In the case of Traveller, I think it was the advent of Traveller: The New Era in 1993 that precipitated the biggest and most lasting schism in the game's fandom, as TNE introduced both a new rules system and effectively rebooted the Imperium setting with which the game had become so strongly identified. I don't think Traveller fandom ever really recovered from the launch of TNE and, indeed, the entire subsequent history of the game and its setting can be seen as a series of course corrections, modifications, and outright backpedaling in relation to what GDW did in 1993. I think it could be argued that the launch of Hero Wars (now HeroQuest) in 2000 had a similar impact on RuneQuest/Glorantha fandom, but, not being plugged into that community, I can't be certain if this assessment is correct.
But, beyond these three games, how many other RPGs have both sufficiently changed over the years and are still played by enough people that there's even a need to distinguish between their current incarnations and their "old school" ones? As I understand it, there are some disagreements among Tunnels & Trolls fans about the merits of editions after the 5th, but those disagreements are comparatively small, owing perhaps to the relative smallness of T&T's fanbase. I also vaguely recall that there was a minor split in GURPS fandom between those who favored pre-Compendium GURPS and those who did not, but I have no idea whether such a split continues or if it was ever enough of a concern to make talk of "old school GURPS" meaningful. I'm sure a similar kinds of minor split exists in the Champions/HERO community too, though, again, this is an outsider's conjecture.
No one speaks of "old school Space Opera" or "old school DragonQuest" or "old school Call of Cthulhu" and for good reason. These, and other games like them, are either so little played in 2011 that no one cares to make any distinctions between "before" and "after" in their histories or they've changed so little over the years that such distinctions are impossible to make. This is why "old school," as a term, largely remains the province of Dungeons & Dragons: it's one of the few RPGs that has a large, active base of players and has experienced a number of major changes, both mechanically and philosophically. It seems to me that these are the primary ingredient for the acknowledgment of an "old school" within a game's history and they're ingredients most RPGs simply don't possess.