Why is the reward of the Prophets greater than that of all other human beings, even though they are infallible regarding grave sins?

Prophets are infallible regarding major sins and minor sins that indicate baseness. It is permissible for minor sins that do not indicate baseness to occur from them, but they are not condoned for them.

Infallibility does not mean a coercive abstention from sin; rather, it is a grace from Allah that inclines them towards good and restrains them from evil, while their free will remains. Therefore, they are rewarded for abandoning disobedience.

Prophets are the most perfect of people in their avoidance of desires, their acts of obedience, and their propagation of religion. They have demonstrated the highest degrees of worship and sacrifice. They are chosen by Allah because He knows best who deserves to be chosen.