A real geologist should give a real answer, but more or less it is due to how the molten elements and molecules sorted out back when the earth was younger and hotter. It has a lot to do with relative densities, melting points and propensity to mix with other materials. Everything heavier than helium and trace amounts of lithium are "star dust", the geology of earth is how that coalesces together and then combines and separates out in geological processes.