I think it was more a matter of how QfG fit so well the graphical adventure style of gameplay. You know:
some rooms available,
you find inventory items to finish quests,
you do so and open some more rooms,
repeat ad infinitum.
It's the cycle you can see an all graphical adventure games, though QfG added a few nice RPG elements to the mix. But it all came down to inventory, rooms, and using items: the hallmark of a graphical adventure.
Mind, I'm not knocking graphical adventures. The QfG series was one of the best, along with the others I've mentioned. But to be an RPG, a game has to offer a lot more than a few different ways to solve something. It has to offer a whole range of simultaneously plots, and many different ways in effect to win.
