In order words, why is voting mandatory in Australia, but not UK, US, Canada or New Zealand?

Maybe it should be mandatory in the US. It would make voter suppression harder, if not impossible.

Another way for conservatives to warehouse people who voted against them in prisons? I’m surprised it’s not already law.

Mandatory voting would include prisoners.

Not in the US, conservatives would make not voting a felony and use it disenfranchise people, otherwise it would never become law to begin with.