Iodine molecule I2 only intereacts with othe iodine molecules by weak interactions suchs as wan der val forces. The carbon that composes diamond is bonded to each other by covalent links, however they form a lattice structure and not a molecule. Since melting temperature is related to the energy necesasry to break inter molecule forces, is is obvious that weak forces such as the iodine molecule will require lower temperature while diamond structure bonds require higher temperatures.
