An answer that is somewhat at an introductory level would be ... Because of the relative strengths of the corresponding acids in the Bronstead reactions of the anions (sulphuric versus nitric). You could explore this explanation in detail in any freshman-year general chemistry textbook. An answer that goes more toward first principles would be ... Because the differences in Gibb's energies of the two reactions. You could explore this idea in far more detail in junior to graduate level physical chemistry textbooks.

Do you have some idea from this answer what you are otherwise really trying to understand about your system?