Because a man's equipment is more obvious and most normal women's swimwear are not made to show their genitals bulging out. Unless one is at an adults-only gathering where the emphasis is on a sexual vibe like Hedonism Resort or a private pool party, I don't think it's appropriate for anyone, male or female, to wear a minimalistic bathing suit that is no more than a strip of cloth with their kibbles & bit and both butt cheeks prominently visible or hanging out only to be seen by others who might not necessarily be comfortable with seeing them.
This apples & oranges double-standard argument is stupid every time it comes up as there is no comparison between the two. If you are at a place that primarily caters to a gay male cliental it is one thing, but news flash...other guys who are straight and the majority of women do not want to see men wearing banana hammocks in public with their schlongs on display, especially when children are or might be in attendance.
Now I do agree that there is a legitimate double standard in the length of men's and women's shorts and by extension, bathing trunks. Back in the day men could get away with showing a little upper thigh while fashion wise, it has become almost unacceptable to have men's swimwear that go above the knee (and sometimes not even the knees are visible) and anyone who reveals their quads may be made to feel out of place. So while an argument could be made there, but as far as something that shows the entire outline of one's nuts and bolt at a public resort, hotel, pool or beach; it is never appropriate IMO.