Because the player is unique in that they can freely switch. In game a priest would either be Holy - Disc - Shadow for their entire life (representing Light - The inbetween - Shadow)
Shadow priests are also not exactly welcomed and seen in a good light, as is made evident by the first campaign quest with the shadow priest rambling off in a corner that you investigate.
It is like the warlock. There is an uncomfortable evilness around them and people will avoid dealing with them if possible but they are tolerated because they have saved the world over and over again.
Also the title is High Priest I believe.
http://legion.wowhead.com/title=482/...ard-from-quest