It has always interested me that many of the Christian saints and feast days seem to have been coopted from pagan rituals that predate Christian faith. My take has always been that this was good salesmanship... easier for the pagan folks to shift to the Christian symbolism and faith.
I think that this made it easier for Christian cultural ideas to be accepted by pagan converts, in the mainstream, and practically speaking it prevented riots or animosity toward the religion. Before Constantine, Christians were persecuted in Rome quite heavily at times, but even after his conversion and the Edict of Milan, Rome was still a pagan city. The process of Christianizing Rome, and the Empire, was a gradual process. I think it was easier to say, "Look, we're doing away with the pagan Sol Invictus feast, but you can still celebrate because it's Christmas for Christians," than "Sol Invictus = bad = no partying for anyone!"