That I can't say. Perhaps Perth was a much better equipped airfield. If you're going to hang around in the air for two hours you may as well get somewhere better able to handle your emergency. The chances of a dual engine failure on an ETOPS-240 (240 minutes single engine operation) certified plane are very small.

Perth isn't an airfield, it'a a major city :) For comparison, Learmonth/Exmouth has a population of ~2000 people, Perth has a population of 2 million.

If the other engine goes, odds are they're not going to make any airport, even the nearby one, even if they're circling it. If Perth has better facilities for handling the problem, and it's "we'll make it there or we won't make it anywhere", might as well go back to Perth.