The American organisational psychologist, Edwin A. Locke pioneered the goal-setting theory. The simplest and most direct explanation of why some people perform better than others is because they have different performance goals. I think that nothing intelligent has ever come from intelligence alone, and nothing reasonable has ever come from reason alone. The essence of the theory is:

Difficult specific goals lead to significantly higher performance than easy goals, no goals, or even the setting of an abstract goal such as urging people to do their best.
Holding ability constant, and given that there is goal commitment, the higher the goal the higher the performance.
Variables such as praise, feedback, or the participation of people in decision-making about the goal only influence behaviour to the extent that they lead to the setting of and subsequent commitment to a specific difficult goal.