Why does it matter so much that the boy is called John, and not Zacharias?

Luke doesn't say what the name means, and these notes give it no meaning. What Luke shows is where the name came from. The angel told Zacharias, "you shall call his name John" (1:13). On the eighth day "they would have called him Zacharias, after the name of his father" (1:59). His mother answered, "Not so; but he will be called John." (1:60). They said to her, "There is no one among your relatives who is called by this name." (1:61), and made signs to his father (1:62). He asked for a writing tablet and wrote, "His name is John." (1:63). The angel had said Zacharias would be silent "until the day that these things will happen" (1:20), and as soon as he had written the name, "His mouth was opened immediately and his tongue freed, and he spoke, blessing God" (1:64). The name came from the angel, not from the family's names, and all who heard "laid them up in their heart, saying, “What then will this child be?”" (1:66).