Why does silicon dioxide have a higher melting point than sulphur?

Silicon dioxide (SiO2) has a macromolecular structure. This means that it forms thousands of covalent bonds between its silicon and oxygen subunits. Sulphur trioxide (SO3), meanwhile, has a simple covalent structure, with no other bonds between molecules. Where SiO2 has strong covalent bonds between molecules, SO3 only has weak van der waals intermolecular forces holding its molecules together. Since covalent bonds require more energy to overcome than van der waals, SiO2 requires a higher temperature than SO3 to melt, thus SiO2 has a higher melting point.