No need to go through a comitte or build consensus when there is only one person that needs to make a decision. On the other hand that often means the person making the decision doesn't need to consider the opinions of others when making decisions.
EDIT To clarify my comment I didn't say that dictatorships are better at making good decisions or result in a more efficient society simply that they are more effectient at making a decision. Whether they make good or bad decisions is much more dependent on the leader. It is a common pattern in the world the more constraints on a position the less autonomy an individual has the less impact they can have either for good or bad.
Probably is related to the fine line between idiocy and genius.