So there might have been a simpler way to give Merlin power: angels. Merlin—and Arthur, for that matter—could be the son of an angel. It certainly would have simplified the entire process. Arthur’s divine mandate could come from his angelic heritage instead of from a sword in a rock.
So why didn’t it?
Well, as I said, we’ve had hundreds of years of Christianity by this point. And interpretations of scripture have become dogmatized. They are interpreted up to a point and then they become fact. And one of those facts is that angels cannot have children. Yes, I know we did a close examination of Genesis before the flood and found out that the Nephilim are a thing. It doesn’t matter; they’ve been edited out. By the Middle Ages, angels are sexless beings. Think Alan Rickman in Dogma. It is an impossibility for angels to conceive children.
Part of this is because of a giant push in early Christianity toward celibacy. Sex in all forms was viewed as bad. This was not directed at certain people, but was everywhere for a time. All of Christianity abstained from sex because it was viewed as impure. Fortunately, some people realized the idiocy of such a position before all Christians died out without having subsequent generations to replace them.
But the prohibitions on sex remained, and were even transferred to the angels, who, as servants of God, would be too pure for something so base.
But devils were another story.