There are two accepted factors that play a role, although they may not be the whole story.
First, Australia is huge and mostly unsettled/wild. Unlike much of the world people haven’t (yet) totally changed the environment to suit themselves, and that includes killing the stuff they don’t like. Large parts of Australia are also warm most of the year, and that drives biodiversity.
Second, there’s a theory that when continental drift turned Australia into an island, the population of reptiles at the time was entirely venomous. Australia is a pretty harsh environment, so those animals thrived and never lost their venom, and there was no parallel population of non-venomous reptiles to diversify their genetics.
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