Here's the thing: President Lincoln had absolutely no way to actually enforce the Emancipation Proclamation. It was a mere gesture.
Now, he had his reasons for making such a gesture.
For one, Lincoln hoped that, when the slaves heard that they had been granted their freedom, the sudden wave of freedmen, as they would come to be called, would help disrupt the war effort.
Perhaps some of these freedmen would join the Union army. That was another small reason.
As for why he didn't extend the Proclamation to the entire country...well, the thing was, he planned to.
Lincoln's greatest ambition was to free the slaves. But even in the North, there existed strong racism. Plus, some Northerners had slaves too, and Lincoln needed the North's support, not only to win the war, but also to support the Thirteenth Amendment he planned to propose after the war ended. This Thirteenth Amendment would make outlaw slavery in the United States forever.
The garrisons were an anti slavery group who helped free other slaves
