You’ve just mentioned one of the things that has annoyed me for many years.

The koala’s diet is mostly Eucalyptus, quite toxic. So they sleep for 20 hours a day, and are slow. This didn’t make sense to me because it would be a sitting target for predators.

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I heard on a podcast once some hunter say Koala’s taste horrific. Then everything clicked. Koala’s eat so much toxic leaves that they become toxic, and so predators avoid eating them.

Evolutionary mystery solved.

The fact they have to spend a lot metabolism against toxins is probably why Clamydia ravages them.

There are barely predators in Australia.
Apart from the Crocs, there's dingos, which aren't naturally occurring there. 
That's pretty much it.