Because as someone said above, women are more bouyant than men, because they have more fat (I know, sad but true).

Because of this bouyancy, a 60kg female with a decent technique will only actually be pulling 15-20kg of load each stroke. There still is a considerable difference in men/womens swim times in professional swimming, but I'd say that the men in triathlon, particually ironman, are no where near developed enough in their upper body to put significant time into the top women swimmers (no point having huge lats in a 50min swim/8-9hour bike/run race).

Mark