He did not free the slaves immediately because he was hoping to find a way to get the Confederacy to come back into the union. He knew that if he did free the slaves immediately, there was no way that the Confederacy would agree to any terms. He also believed that economics would eventually cause the south to give up slavery. He always believed that slavery was immoral.

Its like, right now, why don't we just let felons vote already, their the ones all too familiar with how corrupt the legal system is. I don't think anyone has the power by themselves to change it all at once, there has to be a cultural change. If felons never get rights after they've served time, eventually almost everyone will become felons. The system is self feeding. Many people know whats up and can speak out but it cannot be changed until the majority are convinced. No single person has the power, even by executive order without a large percent of the country backing them up. Even the president would wind up with a ku-de-ta.

He didn't have the power to do that and the whole economy had to change at once or the ones not owning slaves would become slaves. So it had to be a nationwide even all at one time to equalize the economic impact.
Thats my theory.