I like to imagine in part that the Great Valley just happens to be in a prime location to make it the best spot. It tends to be closed off from predators, and the weather doesn't often threaten the residents in a way that would make them need to leave for extended periods. When the Farwalkers in VII are mentioned they seem well known, as if it's like. A consistent thing that they stop in the GV on their travels. I think it was the similar case in XI but it's been quite a while since I've actually seen that one. Some valleys have harsh rainy seasons, causing land floods that they need to travel away from until the wet season is over, some have harsh summers that might dry up the food, or winters that leave all food sources bare. But the valley is just in a prime location and thats why it was in particular so sougth after. (Nevermind indications to the contrary, shhh, I just like to imagine).
Ok, to be honest it never seems they give a good indication of what really makes the Great Valley better than any other green spot but...