Sunday night is Christmas Eve. The SNF game was moved to Monday afternoon (Steelers@Houston)
There are logistical issues with moving games between 1PM and 4PM on a Sunday. More logistical issues with moving it from 1/4PM to SNF, and still more with moving a game from Sunday to Monday. Factor in the Christmas holiday and it's not worth the effort for the NFL and host cities
CBS and Fox can veto a number of regularly scheduled games from being Flex'd to NBC. No way CBS was going to let Patriots*/Steelers get away from them