The slaves had to wait because the Emancipation could only be enacted on enemy territory. Lincoln was limited by Constitutional boundaries, and the Emancipation was justified by war-time powers of the Commander-in-Chief. Lincoln waited because the individual states still in the Union needed to outlaw slavery themselves (most already had anyway), which they promptly did through the 13th Amendment. And it happened during Johnson's term because in the meantime of the Amendment's passing and its ratification, Lincoln was assassinated. Do you even history, bro?