the tau are a technologicaly adavanced race who devote every battle, murder and soldier to "the greater good."
They have no access to melee, and so their shooting is stronger as compensation- so when you are facing off with an enemy shooting army where neither of you can usefully leverage melee as a tool, the Tau are just advantaged.
They promote a progressive ideology. The Greater Good works, is harmonious, and overall seems to be the best system to live under. The Tau "bringing peace to the galaxy" would be by far the least bloody road taken