I understand that the cultural proximity between Canada, Australia, New Zealand and the UK helped to maintain the monarchy in these states, but why is she still the head of state of 12 more states?
For one thing it is politically very difficult to completely overturn anentire system of government and then transition to something such as a republic. This would likely require a referendum and a large scale restructuring of the government apararus. It would also be fairly expensive to so... the answer is probably "mostly inertia"