Why shouldn't the Speaker of the House be a Retired Judge or a current member of the Judiciary, appointed by the Judicial Executive.

This is a really interesting question Peter. 

Appointing a retired or current judge to the position of Speaker would be against the principle of the separation of powers. This principle says that the power to govern should be divided between different bodies to avoid any one branch having too much power. In Australia power is divided between a law-making body (parliament), an executive body (executive government) and the courts (judiciary).

Having a judge as the Speaker of the House of Representatives would lead to the judiciary having law-making power.

Section 35 of the Australian Constitution states the Speaker of the House of Representatives must be a member of the House of Representatives and be chosen by the other members of the House to be their spokesperson and chair its meetings.