At the risk of sounding liek a complete woman-hatrer, the reason Japanese women wear short skirts is because they are better than American women. Japanese women know what looks good and how to pull it off, wheras women over here in the US are...lacking in the fashion AND the "good-looks" department.
This isn't their fault. It's just the fault of both their parents (For raising them that way and for passign down their genes) and the media ("Oh this is so in, bla bla, blah."
There are a few decent women here, but they are either
A. Taken
or
B. Shut-ins
All in all, Japanese women wear short skirts because they CAN pull it off.