Because it took a constitutional amendment to end slavery, which Lincoln helped get through Congress and to the states for ratification before he was murdered.
So, on the one hand, Lincoln argued that the southern states were still members of the Union - and yet you said that it took an Amendment to the Constitution to end slavery. So, why again did he “free slaves” in states that he claimed were still under the jurisdiction of the Constitution, and yet not free those slaves in the states that he actually had a legal and the practical means to do it?
Either way you go, Lincoln did nothing with the Emancipation Proclamation. He freed no one. It was a political stunt, and that is all. It was a stunt to keep England out of the war.
Sic semper tyrannis!