Because its the Cowboys...simple as that
The Cowboys could have been up 56-0 and FOX wouldnt change out the game. Lots of folks are pissed
Also, as mediocre as the Cowboys are, that game was critical to their playoff hopes, as it was to the Rams'. Indeed, the Rams were eliminated with that loss. 49ers-Falcons, while a more exciting game, didn't have nearly as much riding on its outcome. Some viewers might have been upset, but even if they turned off the TV midway through the 4th quarter after Fox didn't switch games, that would have a negligible effect on the overall rating of the game and zero effect on ad revenue.