Renee Duncan tells us that, "In addition to symbolizing the achievement of mind over body – the intangible triumph of the spirit – the crown can be seen as a representation of a halo, which in turn represents genius.
The original Latin term genius was the guiding spirit of a person, family or place, which eventually became represented in art as the halo. Where we generally see angels and saints sporting halos, the crown becomes the representation of genius in rulers. That is, the union with the guiding spirit will help the ruler to be wise."
Wikipedia tells us that a crown, "represents power, legitimacy, victory, triumph, honor, glory, immortality, righteousness and resurrection."