Because some design cues encourages degenerate playstyles.
Easy example : Vengeance in Mists. Taking damage meant tanks got a boost in AP. The more damage taken, the bigger the boost. In other words, Tanks were actively encouraged to take more damage, which is counter to what tanking should be. It became a mini game of using all the tricks in the book to take more damage without dying. Sitting, turning your back to bosses, dropping mitigation, etc…