Because it wasn’t politically, strategically, personally or practically possible. In fact, the Emancipation Proclamation freed no slaves at all, as in September of 1862 no seceded state was yet under Union control.
First, consider the affect it would have on the Army. Most of the soldiers who enlisted by choice did so because they believed they were fighting for union, not the freedom of slaves. If it officially became a war about slavery, then it would be much harder for the Union Army to find recruits or even soldiers willing to aggressively fight. And let’s not forget that while abolitionism was most prevalent in the north, it was by no means the dominant belief, and this was reflected in Congress as well.
Lincoln’s gravest concern was the status of the Border States, Maryland especially. It was likely that the state would erupt into violent opposition if emancipation had been applied to them too. Lincoln had already made more than 2000 unconstitutional arrests, including the Mayor and half the City Council of Baltimore when they began to discuss the possibility of secession, then he had threatened the Supreme Court after they called him on it. Clearly his physical control of the state was precarious to say the least. Lincoln wanted nothing to tip the balance and leave the capital as a lonely island between two seceding states.
Lincoln’s priority was restoring the Union, as stated by him repeatedly throughout the conflict. Abolition had to be his secondary concern. In issuing the Emancipation Proclamation, he appeased his abolitionist critics who viewed it as a legal promise to abolish slavery once victory was achieved, helped to maintain control over the said border states (or at least did not hinder), and it attempted to foment rebellion on southern plantations through what he hoped would be a flood of runaway slaves headed for the Union territory, or even passively aiding the northern effort. This did not happen, as most slaves did not even hear about the Proclamation until much later in the war, if at all. It did affect northern free blacks, however, who saw a Union victory as a victory for abolition, and this led directly to the successful recruitment of 180,000 black troops by war’s end.