Male audiences and the fact that mens tennis is so big. Any stats?
because it's not just women/girls watching it. men come for the view and often stay for the game as they're into it anyway having been familiar with the mens game (beach volleyball doesn't benefit in the same way).
if young women played other sports in dresses, skirts and the stars were good looking enough, the ratings might increase simply due to the male audience tuning in. female tennis players still look like women, some are attractive and many look like the girl next door (e.g. sharapova, julia görges, etc...). They're not guys with ponytails like in some sports so there is the appeal to the opposite sex.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM