This goes back to old posts. I was taught that December 25 (now Jan 6, because of the Gregorian Calender) was a pagan holiday for thousands of years. When they made up Christianity they wanted to appeal to the pagans so they made a Christian holiday the same day as the pagan holiday. The reason for Nittel is because of the pagan holiday.
As an Aside, the pagan holiday was started from Adam Harishon who made a seuda out of Hakaras hatov to Hashem. As the days got shorter Adam thought that Hashem was destroying the world, but a few days after the winter solstice he saw that the days were getting longer, and the change must be part of the natural cycles that Hashem created. Therefore he made a seuda which was then repeated annually. However eventually as a”z became rampant, this hakaras hatov was warped into a holiday to the “sun gods”.