Because he had no Constitutional authority to. Slavery was legal under the Constitution. It took the 13th Amendment to end Slavery in the United States. The Emanicapion Proclamation only freed slaves in areas in revolt that fell under Federal Military Control. They were legally no longer states and therefore the President, as Commander in Chief of the armed forces, had the ability to issue executive orders concerning the law in those areas.
The fact that you ask the question above means that you have a lot of learning to do.
Hallelujah! Sounds like YOU have some learning to do, Kemosabe.