Repeated dialysis eventually results in dissolved ions being removed from the continuous phase. This reduces electrostatic stabilisation.
Dissolved ions can adsorb to the surface of a colloidal particle. Once the surface charge density of the surface is established, there will be an electrostatic force between the surface and the ionic species in the proximity to segregate positive and negatively charged species. This has the effect of enhancing electrostatic stabilisation. However, if dialysis is repeatedly carried out, the concentration of ions in solution (i.e. not absorbed at the surface) eventually tends to zero, so this extra stability is removed.