Why wasn't Jesus named "Emmanuel," as the angel told Joseph?
The short answer
"Emmanuel" means "God is with us." Here "name" means what people would call him, a title describing him, not his given name, which was Jesus. Emmanuel signals that the Messiah would be God with us.
Emmanuel means "God is with us." Matthew recalls Isaiah 7:14 and says it is ultimately fulfilled in Mary's Son: "Behold, the virgin shall be with child and bear a son, and they shall name him Emmanuel" (Matthew 1:23).
"Name" here does not refer to the name Joseph and Mary were to give their son (cf. Matthew 1:21; Luke 1:31). It is used in the sense of "to call": people will call him Emmanuel (cf. Isaiah 7:14). By analogy, one might say of Babe Ruth, "They called him the Sultan of Swat," without meaning that was George Herman Ruth Jr.'s given name. As that nickname marked him as a great hitter, Emmanuel marks the Messiah as God with us.
It also recalls the last verse of Matthew: "And behold, I am with you always, until the end of the age" (Matthew 28:20).