Is this cultural? Or biomechanical? Both perhaps, but I really think that women have a wider variance in their biomechanics, so that there are a very few who can run really fast (say mid-2:20s or faster). Whereas with men, they're built more or less the same, so more can approach the faster times. If this makes any sense.
I can't remember ever hearing this physiological explanation before but it does make a lot of sense. I have no doubt that part of the situation is cultural but the explanation you offer does make enough sense for me to believe that part of it could be physiological.