Why isn't the Gloria said during Advent?

The Gloria is traditionally omitted during Advent to emphasize the season's themes of preparation and anticipation for the coming of Christ at Christmas. Advent is a time of reflection and penance, focusing on the longing for the Messiah, which contrasts with the joy expressed in the Gloria. This practice helps to create a distinct atmosphere for Advent, setting it apart from the celebratory nature of the Christmas season that follows.