I attend daily mass and the day after Christmas we celebrate the feast of St. Stephen the Martyr. Later in that week we celebrate the feast of St. John the Apostle. Why do we jump around so much at Christmas and lose the focus on Jesus' birth?

Yes, and to add to your reflection, we also seem to move forward and backward in time during the Christmas cycle. The Feast of the Holy Family, celebrated on the Sunday between Christmas and New Years features a Gospel of Jesus at 12 years of age. And then at Epiphany, celebrated more than a week later, Jesus is back to being an infant. Further, we observe the Feast of the Holy Innocents on December 28, a terrible slaughter that took place after the visit of the Magi, and then we move backward in time to celebrate the feast of the Epiphany on the Sunday near January 6.
 
Some of these anomalies are explained by the fact that the liturgical year did not develop evenly over the centuries. The feasts of St. Stephen the Martyr, and St. John the Apostle are very ancient feasts on the Church’s calendar. The celebration of Christmas, and the feasts related to Christmas, developed in later centuries.
 
It surprises many of us moderns that the ancient Church did not focus a great deal on the birth of Christ. We are very sentimental about Christmas and the baby Jesus. But the early church focused primarily on the Paschal mystery of Jesus’ passion death and resurrection.
 
In later centuries the Christmas feast became more elevated. But the focus was still more theological than sentimental. Thus, It did not seem so alarming that the very day after Christmas, we were back to celebrating other saints such as St. Stephen and St. John.
 
Later, as a celebration of Christ's incarnation deepened, theologically and culturally, there was developed the octave of Christmas. But the Church did not feel free simply to move aside the feast of St. Stephen and St. John, which were very ancient.
 
As for the chronological whiplash of moving back and forth in time, within the Christmas feasts, we should recall that in the celebration of the Sacred Liturgy we access eternity, rather than merely chronological time. For God, all times and events are equally present, and we meet him there, rather than simply on our schedule.