From a language perspective the Thai crews are undoubtedly competent at English, albeit not on a level that native and colloquial terms don't end up with them all lost.

As far as logistics go I presume that their first flight on any tour of duty is BKK-SYD/MEL, so they are then well placed to operate to wherever, and they work back to BKK at the end of a tour. Throw in minimum rest, possible use of shared rooms, possible use of cheaper hotel - and it all adds up to Thai crews being cheaper.

In terms of having US crews - there's neither a need for them, nor a cost benefit of having them. The only situation where they might be sensible would be on the LAX-JFK run, but realistically it makes no sense.