I don't want to turn this into a flame war about gender, but this does not seem obvious to me. It's certainly not the case in other fields. For example, the best mathematicians are predominantly male. (No female has ever won the Fields Medal or Abel Prize.) The same goes for computer science (only two women have won the Turing Award).

I'm not saying there's no inertia or social dynamics involved, but publicly-traded companies have large incentives to select the best board members they can find. Considering these incentives, a massive imbalance would make sense if one gender was better at running public companies.