A thought occurred to me that it's more likely that a brain initially develops near major sensory input channels. Dinosaurs did have secondary brains thought but their function is also proximity based (ie reflex time gets too long for just one brain location in that sized body).

I could guestimate that the primary constraint is eye to brain distance and the other senses are near because of lessened wiring and easier time integrating the information the nearer to a pointlike approximation one gets. That is if the information itself doesn't require a spesific placing.