Sheer numbers. Numerically speaking far more boys get involved in sports than girls. Also boys typically (but not always) are stronger and faster than girls. I'm not being sexist here, that's just a biological difference in muscle development. But strength and speed alone do not necessarily amount to athletic superiority.
I think it goes back to when we still ha to hunt for food men are engineered to be hunters typically have more stamina than women. Just my opinion though I don't know if it's actually true just a thought