Money, of course - expectation of higher profits
At the next level down:
1. Peace. Long periods of peace promote trade. The past 65 years have been the longest period without major war in many centuries
2. Lower transportation costs - cheap oil, containerization, GPS navigation, etc.
http://en.wikipedia.org/wiki/Containerization
3. Lower communication and management costs
4. Tariff reductions around the world - WTO etc.
http://krugman.blogs.nytimes.com/2009/11/07/us-tariff-history/
5. Technology: to increase the productivity of the unskilled; increase economies of scale; etc.
6. U.S. subsidies for off-shoring
http://www.exponentialimprovement.com/cms/offshoresubsidies.shtml
(Yes, I know you are from Australia, but what the U.S. does affects everyone. For a classic example, the U.S. opening trade with China back in 1971)