The NFL implements a flexible schedule in the second half of the season that prevents any matchups that might have looked interesting before the Season but has since lost some of it's luster to occupy the National spotlight that a Sunday Night game on NBC brings. The Steelers were scheduled to play the Chiefs on a Sunday night in Week 12, and according to Brian McCarthy, the PR guy for the NFL, they will not be flexed out for a different matchup.
For a game to be swapped into the Sunday Night spot the game had to originally be scheduled in the 4 pm window on Sunday. That means the Steelers Vs. Chiefs matchup beat out Redskins Vs. Seahawks, Bears Vs. Raiders, New England Vs. Philadelphia and Denver Vs. San Diego. There are a couple of interesting matchups in there, particularly the Patriots Vs. Eagles and Chargers Vs. Broncos, but it might have just been easier for the League to keep the Steelers game in that spot instead of mixing things up.