A recent review of my game wondered why I had chosen to slap a copyright on my work, rather than opening up my game with an Open Gaming License or Creative Commons license. This was something that I had pondered when I was releasing the game, but I ultimately decided against it.
For one thing, just because other people can reference my work is no guarantee that they will (or won't). Back in the early days of the Open Gaming craze, Gold Rush Games released their Action System as a universal game engine released under the OGL. It failed to take off and Gold Rush Games is no longer a going concern.
And since I'm just a guy with a dream, I had no expectations that I would make a big splash in the world of gaming (and my sales numbers are bearing that out). Certainly not enough to hitch my wagon to the idea of everyone being inspired by my work.
The other reason is quality control. As many long-time D20 players will tell you, there's a lot of crap out there for the D20 System. And I do mean crap. Because when the OGL was unveiled (back in the year 2000), suddenly everyone and their dog (especially their dog) tried to make a million dollars riding the coattails of the D&D brand.
Having a single official source for AiO material means that you can expect consistency (if not quality) out of the product line.