This seems like a more complicated process than you would think, but I read that it is in the overall contract with NBC, ESPN and NFL Network that the disparity of games taken from CBS and FOX can not be more than three, and if they left the Ravens in that slot the tally would be 25-22 in favor of the AFC, which would give NBC no choice but to take an NFC game in week 17. They went with Bears vs Eagles to remain flexible for the final week.
I was surprised myself the game was flexed, but since they had the option to pick up the 3rd and 4th largest tv markets in the country it made sense.