An article at Scientific American offers some explanations, but nobody knows why with certainty. Maybe it’s simply related to the decline in average human body size that started about 10,000 years ago, the dawn of the Agricultural Revolution.
I’d credit the SciAm author but can’t figure out who it is. A quote:
The way we live may have affected brain size. For instance, domesticated animals have smaller brains than their wild counterparts probably because they do not require the extra brainpower that could help them evade predators or hunt for food. Similarly, humans have become more domesticated.
Discovery magazine looked at shrinking brains in 2010.
Steve Parker, M.D.
Paleo Diabetic 