Think back to the time of the early humans. Neanderthals had a smaller cranial capacity than modern humans. I have a theory on why we can use psi. To put it shortly... Evolution.
As I stated before, Neanderthals had a smaller cranial capacity. During our evolution into modern times, we had more space for our brains. I believe since early humans had a smaller brain, they didn't have the part of the brain where the psi is stored and is able to be accessed. If we know that Neanderthals' brains were undeveloped, this would prove my hypothesis that Neanderthals could not use psi. Please comment below! I am looking forward to reading your comments and answering any questions you might have! ~Manipulator of Elements~