I vote for the consumate politically expediant Politician. In my estimation there were several reasons which when combined pushed Lincoln to the decision of the E.P.
1.) Growing dissatifaction among the northern people with the cost of war especially in human lives without any seeming progress. The developeing attitude of just let the South go and keep their slaves.
2.) Treaty Negociations with England were at a stalemate over abolishing the Slave trade on the High Seas.
3.) The expiration of terms of enlistments of the 2 year enlistee who had enlisted in April 1861 and the 9 month enlistees who had enlisted in Sepetmber of 1862 which repersented 1/3 of the Army of the Potomac.
4.) His wish to incorporate Black regiments into the Union Army to offset those losses. Black soldier did not have families who wrote their congressman about the war and it human cost in dead and wounded. They were expendable.
5.) His uncertainty of how the Conscription Act would be recieved.
6.) The high desertion rate within the Army. Army returns for February 1863 recorded over 25,000 men listed as deserters in that month alone, from the Army of the Potomac. February 1863 was the first return in which deserters were indiviually listed as a seperate classification of those listed as Absent. Desertion was so bad that entire companies assigned to pickett duty would leave in one night. Censorship of all mail was instituted so as to intercept shipments of civilian clothes to soldiers from home to aid in these desertions.
Fully another 60,000 men were listed as furloughed at that same time due to illness, or wounds. Most never returned to duty.