During a war that is quite hostile especially one that involves a civil war where a country is divided into two, it is quite dangerous to try to negotiate. This means that the two sides must send their leaders to face each other and talk to each other. This may be one reason why Lincoln was not ready to negotiate with the Confederacy.
Also, there needed to be a good time to negotiate with the other side. Usually, it takes over half of the war and when one side is obviously failing compared to the other side. Therefore, negotiations were attempted near the end of the war when the Confederacy was failing. The Confederacy did not show signs of wanting to negotiate until the end.