Why is the Philosopher-king the best form of rule according to Plato?

Philosophers were to be those that showed great promise early in their physical and mental training. They were to be unrestrained by familial or political desire-being provided everything they need by public rearing. Thus they were to have no egotistical interests in running the state. Also, of course, they were to be philosophers- those that love wisdom, the seekers of unimpeded truth. Thus, they want nothing more than the well being of the state.