The New England Patriots have already played six games in prime time. CBS saved them from playing a seventh.
NBC owns the right to flex games into the Sunday Night window after week 5 of the regular season, in order to put superior product on the television in case the original scheduling turns out flat. CBS and FOX can protect games to prevent losing high quality games in order to save their own ratings. The week 14 games between the Patriots and the Texans was flexed to Sunday Night Football earlier this season, the Patriots sixth prime time game of the year.
The flex schedule only applies to games on Sunday, and week 16 offers a weak slate. The two best games of the wee (Eagles-Washington; Bengals-Broncos) are on Saturday and Monday, respectively, and couldn't be flexed. Sunday offered just the Patriots-Jets (on CBS), the Panthers-Falcons (on FOX), and Packers-Cardinals (on FOX).
The Patriots-Jets was protected, and that meant that FOX protected one of their two games. While there is national interest for the Panthers, the two NFC South teams rank towards the bottom half of the league in national appeal, while the Packers rank 3rd. It's likely that FOX protected the Packers-Cardinals, knowing the small appeal of Panthers-Falcons, and understanding that a potential 16-0 Panthers game is more likely to be flexed than the 15-0 game.
Personally, I'm relieved that the Patriots and Jets will play at 1:00 PM. Early afternoon games are just far more enjoyable; I don't like working into the wee hours of the morning, and it makes waking up for work on Monday a real Mark Sanchez into a Brandon Moore.
A Patriots victory locks in New England for homefield advantage.
12/22 10:14 am edit: The NFL gave the following response when we reached out for information
"Per the NFL Broadcast Policy, each team is limited to five scheduled primetime games in one season, or six if they are flexed into one Sunday night matchup."