The isoelectric point is the pH at which the zwitterion exists which contains both the negative and positive portion of the amino acids. Fo aspartic acid, the isoelectric point is 2.8 because it is an acidic amino acid and contains a carboxylate group. Because of this, the H+ concentration will have to be very high (i.e. low pH) to avoid deprotonation of the amino group ion (NH3+) by the carboxylate group on neighboring molecules. At low H+ concentrations, the H+ ions of the amino ion group will be removed. For leucine, the side chain is nonpolar so it doesn't have the same attraction towards the H+ from the NH3 group so we do not have to have excess H+ to form and/or maintain the zwitterion.
The difference in the side chain results in differences in the way the molecules interact with one another and therfore the amount of H+ needed in solution to achieve the zwitterion.