"And they said, why is there not sent down to him an angel," meaning: one who would command us to follow him.

Allah says: "But if We had sent down an angel, the matter would have been decided," with their punishment, "then they would not have been reprieved," meaning they would not have been granted any delay after the descent of the angel; for when a people ask their prophet for a sign and it comes to them but they still do not believe, Allah destroys them.