Why is sulfuric acid stronger than sulfurous acid?

The strength of an oxyacid is determined by the extent to which it dissociates in water to form hydrogen ions. For a given nonmetal central atom, acid strength increases as the oxidation number of that central atom increases.1

In the case of sulfur-based oxyacids, sulfuric acid ($H_{2}S{O}_4$) is a stronger acid than sulfurous acid ($H_{2}S{O}_3$) because the sulfur atom in sulfuric acid is in a +6 oxidation state, whereas the sulfur atom in sulfurous acid is in a +4 oxidation state. This higher oxidation state in sulfuric acid contributes to its greater ability to donate protons in solution.1