Australia is an island. It takes a lot of effort to get there. Canada has only one neighbor and very few people are waiting to get out of that neighbor.
United States shares huge land borders with Canada and Mexico and is in very close proximity to Cuba, Haiti and other Caribbean countries. Other than Canada, the economic levels of neighbors of the US are substantially lower than that of the US - leading to greater demand to migrate.
Majority (90%+) of Australia and Canada are uninhabitable. More than half of the USA is habitable and there are plenty of areas like California and Florida that are quite comfortable to a lot of people.
In terms of GDP and job opportunities USA beats almost every country on earth.
USA has a huge immigrant population and these act as safe places for new immigrants to come in. Almost all ethnicities in the world have a sizable representation in the US - leading their family, friends and other associates to move in with relative ease [first on visa and then overstaying].