Because it wouldn’t have passed. 3/4 of states needed to ratify. If you think we are divided today. It was by far much worse then. When Lincoln was elected they left the union.
Earlier in your post 117, you wrote:
“Politically it also became clear early on that the Union could not be put together the way it was. Emancipation was the only way.”
Let's combine your two thoughts: the North, ostensibly, wanted to “free the slaves” but couldn't do it peacefully because they didn't have the votes required by the constitution that all the states agreed to so many years before.
If true, what the North needed was a war. But first they needed a pretext for war - perhaps, “Save the Union!”
All well and good but . . . President Lincoln took an oath to defend and uphold the U.S. constitution not to overthrow it and the provisions he didn't like by using the military.
By your telling, that's what he did. And he killed a lot of people in the doing. As you say, he could get the 3/4 vote to ratify and “emancipation was the only way.”