Why does Merlyn focus on educating Wart instead of Kay?

Merlyn is aware that Wart, aka Arthur, is destined to be king. In some versions of the legend of King Arthur, the wizard Merlin is directly responsible for orchestrating the circumstances of Arthur's conception, making sure he is fathered by Uther Pendragon, a powerful warrior who is believed to be descended from noble ancestry that once ruled England.
Merlyn sees it as his responsibility to mentor and teach Wart about things like nature, courage, kindness and ethical behavior. Since Kay is of noble birth, he will be able to become a knight without much difficulty, and enjoy a respected position as an adult. But Wart, whose origins are more mysterious when he is younger, and before he proves himself to be the rightful king by pulling the sword from the stone, is perceived to be of lowly birth. Merlyn understands that if Wart is to become a good king, he needs to be given the opportunity to learn and be exposed to the world in his youth. He attempts to compensate for Wart's humble beginnings by giving him lessons gleaned from his own experience and wisdom, as well as his magical powers.