Augustus is emperor; Quirinius is govenor. The emperor decrees that all persons be registered; that is, the emperor is going to make sure that he gets taxes from everybody under his control.
Then there are some folks who can't issue decrees. The only things they control are somebody else's sheep. And it is to this kind of person that the angels go with their news. Not the emperor, not the governor, but the shepherds.
Although shepherds had a positive image in the Old Testament--think of the 23rd Psalm for example--shepherds living and working at the time of Jesus' birth were not viewed positively. Rather, they were regarded as lower class, untrustworthy, migrant workers who used other people's grass to feed their sheep.
The shepherds were not expecting the news. They were at work, and, to their society at the time, not very well-thought-of work. Yet, the Lord sent a messenger to them with the good news.
The response of the shepherds was immediate. They went to Bethlehem at once to see for themselves. And when they had seen, they told what they had seen.
Think about who God trusted to receive and carry messages. Try to imagine a modern-day counterpart to first-century shepherds. Would you be interested in anything such people had to say to you? Is it hard for you to imagine God's telling them something before letting you know?