One reason is that the dust cloud that formed our solar system have already gone through several rounds of of star formation and supernova to seed it with the heavy elements.

The second reason is that when the sun reached fusion and started pumping out the solar wind it pushed a lot of the lighter elements to further out in the solar system. That is why the rocky planets are the inner four and the gas giants are the outer four.

Earth has a much higher proportion of heaviest elements than the proto-cloud (which was metal rich in the first place) because of the second reason.