I think it also has a lot to do with the size of our country in addition to our population. Smaller fanbase, with wider distribution around the country, means that it's harder to reach any kind of threshold value to make a convention viable.
Plus, when Gok has looked into it, there's a very high fee that Hasbro Aus. charges for something to be given "Official" status, and thus get their support in bringing over voice actors/designers.