Because he said "It's not my job". It was a stupid thing to say. Murray just is not a leader. He is all about himself and always has been. When things are going badly, he points to everyone else. He has horrible body language on the field.
Hey, we can't all be good leaders, but as a QB in the NFL you need a baseline of leadership, or at least don't be a distraction, and he always makes himself a distraction somehow. Sorry, I feel strongly about this. He is a sucky leader.