Answer: It is because of an insufficient supply of NaOH needed for the complete reaction with the diprotic acid H2SO4.
Explanation
Neutralization reaction is the reaction between an acid and a base or alkalis to yield salt and water.
Monoprotic acids like HCl ionizes only in one step
Diprotic acids ( with two protons or hydrogen ions per molecule ) like H2SO4 ionizes in a stepwise manner by losing one proton at a time
Also, a salt is formed when all or part of the ionizable hydrogen of an acid is replaced by a metal or ammonium ion.
Monoprotic acids can only form one type of salt – normal salts
Diprotic acid can form two types of salts – normal salts and acid salts.
The product of the first equation NaHSO4 (acid salt) was formed as a result of the insufficient supply of NaOH hence the hydrogen ions of H2SO4 were partially replaced while in the second equation Na2SO4 (normal salt) was formed because 2 moles of NaOH was used in the reaction and that was enough to completely neutralize the diprotic acid H2SO4.