Gorgon Zee
Hero
But I think you are a bit off on this point; there were several successful uses of the d20 core that were not fantasy based. I would agree if you said that d20 lent itself to heroic RPGs, but its robust enough to support horror, super-heroes, romance, etc. if one feels so inclined.
I'm using a fairly limited definition of success here -- it's building a long-term sustainable product line. There were definitely fun and profitable works based on d20. I ran a Slaine campaign and liked the Dredd works, for example. But nothing really gained long-term traction.
Also, I really do feel that d20 is pretty tightly bound to heroic fantasy. It can be reskinned to "support" other genres, but I am of the school that thinks that rules and genre are strongly intertwined. I know not everyone thinks this, but my feeling is that different genres work better with different systems.
As an example, take Call of Cthulhu. The BRP system used in chaosium's rules (I have not read seventh edition rules yet) is, to my mind, a very weak system. Mechanically much inferior to d20. And yet, I an many others love playing horror using it and CoC d20 was a flop.
But anyway, please don't take my comments as saying that all D20 games were bad, or failed to be profitable. Certainly that is not the case. What I should have made more clear is that they failed to create a sustainable product. Which I believe Paizo have, uniquely, managed to do.