Isoelectric point of aspartate
The -CO₂H groups are carboxylic acid groups, better represented as -COOH. These, being acidic, have a lower pKₐ than the -NH₃⁺ group and so as the pH increases these -COOH groups lose their protons first.

The pKₐ values for the three are shown below the arrows: as you can see the α carboxylic acid is more acidic than that in the side chain, but they are both much more acidic than the protonated amino group.