It really just comes down to clock management. It's not a one score game unless you make the conversion. If you wait until the end, you either burn all the time on the clock and risk it all on the last play, only to find out it was actually a two score game the entire time and you just didn't know it, or you keep enough time "just in case" which means risking giving the ball back to Atlanta with enough time for themselves to score. If you go for two early, it's easier to work the clock in your favor.