I did not like the decision, but Allen's injury could have factored into it.
A team that's down by 3 on the road will almost always settle for a FG instead of trying to score a TD, whereas a team that is down by 6 of course has to try to score a TD to win the game. If the Bills don't covert, I feel like the worst that can happen is the Cards kick a FG to tie, and in that scenario, it's likely that the Bills, who had two timeouts left, get the ball back with some time left to get the winning score, or at worst you go to OT.