Why isn't Jonathon fully accepted by the Valley people in the novel, Firegold?

Jonathon is a "half-breed," not fully accepted by the Valley people among whom he has grown up because his mother was half Dalriada. The Dalriadas are mountain people who ride splendid horses, possess modest psychic powers and, according to Valley belief, grow horns on their heads. Because of this belief and the fact that there has been almost no direct contact between the two groups for generations, Valley dwellers also believe the Dalriadas are not completely human.