Koalas have no reason to have much brain power.
Koalas aren't good eating - they're toxic because they're full of eucalyptus oil. So they don't need to avoid predators. They eat one thing, so all they need the brains for is to find the thing and eat it.
Having a large, metabolically active brain would be a bad thing for a koala because the food they eat is so low on nutrition they'd be wasting energy running it.
There's selective pressure *away* from having brain power.