There was a really great writeup by someone who worked in that Disney animation shop on a blog I once stumbled on - he wrote about how he took on the 'job of a lifetime' as a teenager while his mother was terminally ill and how that impacts your priorities.

Incredibly moving read that I unfortunately can't find now, but it covered how the Sydney office ended, from memory it was a combination of CGI changing the industry, and cost - Australian wages weren't cheap and the studio was mostly used for producing direct-to-video sequels which was a market that dried up.

The actual big influence on the Australian-American cross pollination though, I suspect, was Rupert Murdoch's expansion into the American film and TV industry. For all his faults, he was a big champion of talent from his homeland in both his movies and Fox TV network. Even the infamous Simpsons Australia episode was likely influenced by the big influence of Australians at Fox at that time.