At the junior level, swimming has by far the highest participation rate of any organized sport followed by soccer, aussie rules and netball
FYI, cricket is played by significantly more kids than most sports and the participation rates are pretty healthy
As a parent, the only way I can analyse the above is as follows.
1. Learning how to swim is essential (virtually every parent I know gets their kids formal training and they really seem to love it)
2. Aussie Rules is an incredibly well administered/marketed sport in this country with Auskick programs everywhere plus it is easy for kids to understand
3. Soccer is marketed brilliantly everywhere and is popular with parents as the chances of little Johnny getting beaten up are less than with union or league
4. Netball is the girls' national sport
5. Cricket is played in summer