For the same reasons jw's reject Christmas they use the name jehovah.
Dec 25 is the recognized day to celebrate the birth of Christ in the world.- we use Jehovah because it is the recognized name of the father in the world.
It Dishonors Jesus because we don't know the actual date of Jesus birth- even though we don't know the actual pronouncing of gods name it brings honor to God to use it
Even though the angels rejoiced at the birth of Jesus, no where in the Bible does it say we should celebrate it. Even though no where in the Bible does it say Christians are to be witnesses of Jehovah, we proudly bear his name!.