The correct answer to this open question is the following.
I think the Emancipation Declaration was not considered legal by many based on the wording because it did not apply to all the states, just the states that had seceded from the Union before the Civil War started. This also applied to some states that already was under the control of the Union army.
Another reason was that the liberty of the slaves totally depended on the Union winning the war. Let's have in mind that the Emancipation Declaration was issued in January 1863, when the war was in its third year.
For many historians, the importance of this document was that it gave African Americans a motive to celebrate and keep the hopes alive.