I heard this somewhere, and I apologize I can't remember where, but the gist is that Vegas is keeping the line low as a hedge against the futures bets placed on the Falcons when their odds were enormous.
I have no idea if it's true or how to verify it, but there was supposedly quite a bit of futures action on Atlanta, and with odds getting as high as 100-1 to 150-1 early in the season, they've got a lot of money riding on New England winning the game.
To hedge, they're keeping the line artificially low so as to encourage betting on the Patriots, which will help ease the sting if Atlanta does win.
Again, don't know if it's true or how to verify it, but figured it was worth noting in case it is.