Simply, the why of Christmas is Jesus.
We on our own could never hope of getting to God, so God took on our skin and came to us.
We could never on our own be good enough for God, so God in Jesus became our goodness by His own perfection.
The Bible says that we were strangers to God’s promises, far away from hope, and as lost and helpless as humanly possible, but God in Jesus came near to bring us near, to give us hope, and to save us from our sins and ourselves.
Christmas is Emmanuel. God with us. God for us. God in us.