Rather than ask why people start celebrating christmas, it would be more meaningful to examine why JWs don't.
We know that the reasoning is flawed and early Bible Students did celebrate Christmas (the old song book in our hall contains Silent night). Rutherford banned Christmas to seperate JWs from other christian faiths to make them different and to exert control. It was one of the USPs of the faith it's contrarinous and holier than thou attitude. But Rutherford;s motives were not whiter than white.
That being said why would anyone want to hold fast to a teaching from a man with Rutherford's track record? The fact that he banned it is even more reason to deck the halls.
Besides it is fun and frivalous and brightens up a dreary winter.