Volkanovsky, Whittaker, and Matthews are each raising families, which makes training away from Australia far more problematic for them than for fighters like Clark and Anderson who are single and without kids. That may have more to do with not choosing to come to America to train than anything else. From what I've heard from Australian fighters, it is not controversial to say that one can find better MMA training in the U.S. than in Australia.
Joanna Jedrzejczyk became a champion by training in Poland, but I don't think that means that the training in Poland is just as good as in the U.S. If Joanna believed that, I doubt she would have relocated her training to the U.S. She's single, and so she trains where she thinks it is best for her career to train. That one can find some champions training in a country doesn't preclude the idea that they may have become champions despite disadvantages.