James Maliszewski over on Grognardia is spot on with his post on the OSR and older edition D&D.
Everytime I see this come up, like on the theRPGsite, I point out the OSR is what it is. Because of it's grassroots origins, the OSR reflects the desires of gamers not the business plans of companies. And the simple fact is that most old school gamers prefer an older edition D&D over other RPGs.
As one of the commentators pointed out many of the other popular old school RPGs are still supported by active publishers. This takes away a lot of the reasons for a grassroots movement to form around those games. And explains why the OSR is so overwhelmingly focused on older edition D&D.
And other commentators point out that large segments of the OSR are involved in preserving other older edition games. Some are involved in creating new games for different genres designed to appeal to old school gamers. Mutant Future, and Stars without Number are two of many examples.
The nice things about all of this is that because of it's foundation on the Open Game License there is very little to stop anybody from publishing what interests them. In addition the expansion of the OSR has lead to opportunities to buy original games from their copyright holders and release them for the current generation of gamers. Spacemen & Spaceships from Goblinoid Games is an example of this.