Because the interaction forces between gas molecules are less strong than in a liquid. But I don't really see why, image two silica particles immersed in a liquid or in a gas.. I actually think they are larger is a gas than in a liquid. In the van der waals contributions there is the contribution of the dielectric constant, which is larger for the liquid than for the gas. The dieelectric constant stands in the denominator. But why does a smaller electric constant benefits van der waals forces physically?