You ask the question, Why?
The basic answer is that the JWs are obedient to the Governing Body who have decided that such celebration is "displeasing to Jehovah" They will say that anytime they are mentioned in the Bible ir is in a bad light. E g. HEROD when Salome danced for the Head of John the Baptist.
There is no specific Biblical verse that says not to do it. The history of their belief goes back to the past President of the movement JF Rutherford who has a record of laying down rules based on his opinions. There is a lot to discuss about him should you wish.
To the rational outsider it makes no sense at all