hehehhe time for my civil war obsession to come in handy
Lincoln needed to announce it right after a big win for the Union. Antietam happened to be a well timed one.
Doing it after a big victory would help reinforce the fact that he was doing the right thing in the war, and that he was a good leader (good enough to be re-elected).
Announcing it right after a loss would be viewed as a weak, desperate move, and Lincoln couldn't afford that if he was to be running again for president.