Why is our governance different from the United States of America?

Answer

Whilst Australia and its sister nations under the Crown and the United States of America are all functioning democracies, there are radical differences.  The US Constitution, which was written in 1787, was based on the system then existing in Britain with the King replaced by a president.  Of course, there were other differences, but the basic structure was essentially the same.

The British system evolved over the next hundred years into a constitutional monarchy with responsible government whilst America remained fundamentally tied to its 1787 Constitution.  The US does not have responsible government. 

Responsible government refers to a government that is responsible to the people.  Members of the cabinet (government) must be taken from the parliament and it, the government, can only operate with the confidence of the lower house of the parliament.

In the United States, the president selects his cabinet from outside the Parliament (the Congress and the Senate) and it is answerable only to him.  In Australia the Prime Minister is head of the government and the Governor-General, as representative of the Queen, is the executive head of state whilst in the United States the president is head of state, both ceremonial and executive, as well as being head of the government.