Probably because once you leave the dungeon, the instance server has no way of tracking you down. I wouldn't be surprised.
I would have loved it if we are given the vote option only after we exit the dungeon. And then it gets delivered to us with a message popping up, similar to the notification when someone buys our items on the market board.
I don't like how we have to stay at the end and never know if people have already given their commendations, or are just being slow. So I've always just left after the coffer has been rolled---and when the coffer contains something I can't roll anyway, I just leave. Supposedly tanks will be swimming in commendations anyway, they say.