Actually, the Bota Magna Vorox are intelligent. I think they were smart enough to live in a resource-rich jungle and build a civilization, while the less intelligent desert dwellers lost intelligence in favor of animalistic abilities due to intelligence being worthless if all you ever do is kill mindless beasts for a meal without the resources to make anything more complex or useful than a crude blade.
So you're thinking basically the intelligent ones migrated to the best land prior to the Shattering, and kinda kicked out their less clever cousins or something like that?
Incidentally, is there such a thing as a mindless beast? People use that figure of speech, but surviving animals tend to be quite clever. And survival in a desert situation can indeed take extreme cleverness. Even humans can die for lack of use of brains in them.
My guess would be, though, considering the lifespan thing, that the desert Vorox might have gotten so used to just doing the certain things necessary to survive that it became more like instinct, and they had no need to hold on to the ability to adapt. Even though finding food and water in that takes intelligence, once you know how to do all that, it could become kinda automatic.