An Angel Visited the Shepherds

The Lord chose to publicly the announce the birth of the Messiah to an unlikely group, to a group of shepherds. And while one angel was good enough to visit Mary, and one angel was good enough to visit Joseph, the Lord sent a "heavenly host" to visit the shepherds. Why did an angel army announce that God was bringing "peace on earth" with the advent of the messiah? (Luke 2.8-20, December 14,2014)