The two most abundant elements in the Earth's crust are silicon and oxygen. Most of the silicon in the lithosphere is bonded to oxygen and exists as silicate minerals. This is because the silicon-oxygen bond is incredibly strong and forms highly organized lattice networks in solid minerals with very high melting points and relatively low chemical reactivity. So when the Earth was formed, the silicon formed silicates with oxygen with other elements mixed in. Over time, smaller elements formed gasses and volatile compounds that left the crust and escaped into the atmosphere (carbon as carbon dioxide gas, for example) while heavier metals sank closer towards the core of the planet. This left silicon and oxygen silicates as the predominant chemicals in the lithosphere.