You've answered a question that has an open bounty. You hope the bounty will be awarded to your hard-worked-on answer. But there are other answers, and some of them are good as well.
If you are awarded the bounty you will of course receive a notification. But if you are not, there's no way for you to know which answer got the bounty. Even checking the "featured" question list won't work: when the bounty is awarded, the question disappears from that list.
So, I think it would be a good idea to notify all answerers of a bounty question when the bounty has been awarded. Most of those answerers probably want to know which answer finally got the bounty.