Lincoln “apologists” (and the rest of us too!) answer that question all the time. The answer is exceedingly simple - because he didn’t have the authority to unilaterally do so. He knew that it would take an amendment to the constitution to free all of the slaves.
See how easy that was?!
So how can you have a War to free the slaves, when it required a constitutional amendment?
By the way, what was the date they initiated that constitutional amendment? Because surely it was before the war, seeing as how that was the whole purpose of the war, wasn't it? Obviously the Union would want to do things proper and constitutional, and of course that would commence with the amendment process, right?
So he did not have the authority to free slaves, but he did have the authority to jail, without trial, the entire elected legislature of the sovereign state of Maryland?
If Obama did half of what Lincoln did you would rightly denounce him as a tyrant. You can thank Lincoln’s unchecked tyranny and several generations of school children educated in Lincoln deification for the destruction Obama has brought to America.