I think this is the wrong question. Why is voting optional? We can crow about nebulous “freedom”, but we are forced to do much harder things all the time. Once every few years, we’re asked to make a few decisions.

If the idea is that the citizens select leaders, but it’s incredibly easy to opt out, what you have is a biased selection committee. We can argue all day about the various biases involved and if they matter, but the reality is that it’s not burdensome on Australians and it actually makes sense to get your source election data as close to based on public sentiment as you can.