I'm willing to bet there are at least double the number of Canadians as Australians.
Canadian players have even fewer good options than Australians as the most talented Australians can turn pro at home. Canadians can stay at home in their college system, but obviously the level of play and the benefits received by being a student athlete are much higher here. Also, Canadians can play the American club circuit and even some of their schools can play American teams so they are essentially part of the same recruiting process. Declining pro salaries are a big reason why there have been more good Spanish players choosing college. Australian pay is not great for the non-stars, but I think part of the decision making process for players there has been the ability to get playing time without having to compete with players with a decade more experience, which is part of the reason that more French prospects have also decided to come here.
I believe what you are seeing is a number of these ladies were young girls when Lauren Jackson was at her peak. Jackson had a lot of injury issues that hampered her career but when she was healthy, she was arguably the best player in the world. Penny Taylor was never LJ but she too had.a very successful WNBA career. Given the age of the traditional college freshmen, it is realistic to think the fact those ladies had such successful careers in the WNBA had some degree of influence on the decision for these girls to play in the US.
That's an interesting thought as Australia and the United States have generally had a mutual respect for basketball in each other's countries, unlike the relationship between the United States and most of Europe. I think part of the reason is that technology has advanced to the point where the recruiting process is easier. It's now getting simpler for players who are not the top prospects to get film from local club play that they can easily send to colleges, reducing the mystery level. Australians themselves generally do not believe that their recent prospects are any better than the relative level of prospects that they had in earlier generations and some of them are actually quite critical of the quality of player development there right now.
If your theory is correct, it will be very interesting to see how things change in the next few decades. Australian Rules Football has become a viable professional sports option for women now and it is the first love of many Australian athletes who have been competing in other sports, including many basketball players. Basketball was already behind netball and soccer and also competing with individual sports, but they may now be even further down the list. Here, basketball certainly competes with volleyball and soccer, but is not behind other sports like in Australia.
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