Oh gosh, I wish I had an answer for you! I can think of several reasons why they are popular here - they are a great way to get cyclists from all over the place together to ride; they are a wonderful fund-raiser for various charities and they are a physical challenge without it being a race.

As for why there are none in Australia - I had no idea there weren't any! Wow...so....someone needs to take the plunge & organize one!