Why do nonpolar substances tend to be gases and not liquids or solids?

Because the intermolecular interactions are weaker.

Explanation:

Nonpolar substances do not have dipole-dipole or hydrogen bond interactions, which are strong intermolecular interactions that often make the substance liquid or solid. Instead, they must rely on van der Waals forces, which are relatively weaker interactions that are often insufficient to hold the molecules together in liquid or solid form.