Because I can't imagine Merlin ever using magic to defend himself: not when Hunith was so adamant that he must keep it secret, and when he knew he might accidentally hurt them more than they deserved. Will, of course, has no such scruples.
His secret was safe, but still the boys in Ealdor could tell that there was something different about Merlin. They used to taunt him for sport, their eyes alight with malicious laughter.
He repeated the words Hunith had taught him, whispering to himself. "Sticks and stones may break my bones but names will never hurt me."
They heard, and they threw sticks and stones at him after that.
He was fine, still, but he was even more fine when Will arrived, with awkwardly muttered words of comfort and the ready fist of a ten year old whose friend needed him.