Dicarboxylic acids have two dissociation constants, one for the initial dissociation into a monoanion and one for

Question:

Dicarboxylic acids have two dissociation constants, one for the initial dissociation into a monoanion and one for the second dissociation into a dianion. For oxalic acid, HO2C—CO2H, the first ionization constant has pKa1 = 1.2 and the second ionization constant has pKa2 = 4.2. Why is the second carboxyl group so much less acidic than the first?