There's a difference between preference and taste. In other words, there is a certain standard of quality that transcends genre. There is good swing music from the 40's, and bad swing music from the 40's. There is good modern rock, and there is Nickelback.
PHDs don't determine a standard of taste, they determine canonicity based on that standard of taste. Those with a broad reading base, that is, find books which are well written, and assemble a canon. They don't determine what good writing is, they recognize it. If anyone determines what makes "good" art "good", it's aesthetic philosophers.