A part of the reason is England not giving Australia preference in trading anymore due to the EEC or rather the European Economic Community as per their regulations when England applied to join. This applied to all commonwealth nations not in the EU so Australia looked to America for their trading since America was offering better terms. Historically trade and culture go hand in hand so our culture has been injected with American culture due to the large amount of consumables coming our way.

Politically someone else is going to need to comment since im no-where near well versed enough to comment