[SunilNagpal]

Why protiens are precipitated at their isoelectric point?

At isoelectric point there is no net charge on a protein.

At any other pH there remains a net electric charge on a protein which enables it to interact with water (or other related solvents i.e polar solvents). So, if a protein lacks a charge, it will not interact with the solvent (assuming it is polar); which in turn will lead to over powering of protein-protein interactions as compared to protein-water (or polar solvent) interactions; thereby leading to precipitation.