Because for every 1 mole of H2SO4, it reacts with 2 moles of NaOH,
H2SO4 + 2NaOH -> Na2SO4 + 2H2O
H2SO4 is diproctic, in the presence of a strong base (NaOH), it forces H2SO4 to fully ionize, neglecting the reversible part of HSO4 being amphiprotic, basically H2SO4 has 2 hydrogens and NaOH only...