Dispersion forces, otherwise known as induced dipole-induced dipole interactions, polarize a molecule slightly into weak negative and weak positive charges. Then coulombs law dictates their electrostatic attractions. However, in dipole-dipole, these molecules are already polarized permanently and typically of a much larger magnitude than an induced dipole for a molecule of the same molar mass. This means that the electrostatic attractions are significantly stronger than the London Dispersion force's.
