Bears scored a touchdown with 0 seconds remaining on the clock. After the 6 from the touchdown they were down 9-21. The extra point kick or two points conversion makes no difference in the outcome of the actual game, so the game ends without an extra point or two point conversion attempt. Now, if that touchdown had put them down 19-21, they would be allowed to come back out and attempt a two point conversion, even with zero seconds left on the clock, since it would affect the outcome of the game. Same thing if that touchdown had made the score 20-21 or 21-21.