The first missionaries about the Messiah were not the apostles, and missions didn't start at Pentecost. Instead, God chose a simple group of people to carry His message that was heralded by angels.
God could have had the angels proclaim the birth of Christ throughout the world. He could have let everyone see the glorious light and the shining star. Yet He chose to reveal truth to a few, who would spread the word to so many more.
The shepherds were no different than you and I, really. Simply people who were recipients of revelation - and they passed that revelation on to others.
"And all who heard it wondered".
Continue to pass along to others what has been revealed to you!!!!