It was his mom, Catherine, now St. Katarina, who was pulling the strings on that; and IIRC she was the one who insisted on Nicea.

Apart from the myth that his mother was a British Christian, it is also possible that the Christians had become a powerful political opposition to the government in Rome, and a merger of the Sol Invictus movement with them would make his leadership unassailable. So the Sun God became the Son of God, and the Winter Solstice became Christmas Day (subject to later calendrical adjustments).