Because if he has declared the slaves free too early in the war, the border stats which had slavery but never left the union such as Maryland and Kentucky might have decided to join the confederacy as well. He couldn't lose anymore support for the war effort
The Declaration of Independence failed to change the status of Americans as rebels, and not soldiers of a recognized nation. If one looks into the distant future, the preamble failed to include blacks, Indians, women and others as equals.
