"Emperor" denotes tyranny much more than "King" or "Supreme [insert title]". Its associated more with dictatorship type of government. The truth is that most people--and I'm sure Lucas himself--don't know that "majesty" or "highness" have specific uses. I sure didn't. Lucas just choses things for their aesthetic sound or associative connotation. Highness just seemed to be a better word to use, and Emperor is more apporpriate for a tyrant.