Because of solar wind. After the sun formed from the material of the proto solar system it started producing solar winds. This pushes light elements to the edge

Because of solar wind. After the sun formed from the material of the proto solar system it started producing solar winds. This pushes light elements to the edge of the solar system but heavy elements stay. So rocky planets form. Then the light elements collect as well and reenter the interior solar system as comets which redeposit light elements on the surface of rocky planets. In the mean time the light elements that collected together in great quantities formed the gas planets. This is all a very traditional view afaik and doesn’t explain where mantle light elements come from. For example there is a great deal of water that is in the mantle that drives geochemical changes in the mantle rocks. Was that there originally? Or was it put their after plate tectonics started and subduction sucked water into the mantle? I don’t know but I would assume there are plenty of geodynamics people who would have opinions more deeper than mine on the topic.