James Maliszewski has attempted to answer some criticisms that the OSR is too "D&D-centric". As he and others point out, there's certainly some activity relating to other games... and we must indeed acknowledge that there has always been more interest in fantasy gaming, and D&D specifically, than in other games and genres.
But there's also something else at work: other old-school games, much like the "Forge-style games", tend to be more focused, either tied to a specific kind of setting or play style. The BRP-based games (RuneQuest, Call of Cthulhu) have changed very little, because their rules sets were designed to hit a specific spot, as was The Fantasy Trip. Even if you ignore the Traveller setting, the game is designed for a specific kind of space exploration setting.
And, in a sense, aren't all the other games D&D as well?