Because guns are crude!

In Episode III: Revenge of the Sith after defeating Grievous with a blaster, Obi Wan comments how 'crude' blasters, presumably in comparison with lightsabers.

Wookieepedia explains that these weapons were:

Designed as much for elegance in combat as for ceremony

If we look at the origins of the lightsaber, they started out as forcesabers through which:

Users [could] channel the dark side of the Force through black laboratory-grown crystals which would create the glowing energy blades