I have no evidence, only speculation, but it could be because boys do not receive the same type of nurture and care growing up compared to girls. As well as, anecdotally, being more likely to engage in dangerous (for the brain) activities and take less care of themselves. The last point manifests itself in many ways, but the most important one I can think of is diet.

Again, this is all conjecture and I don't know how I would go about finding the studies to link it together.