Why? No one will like a truthful answer to this question.
Most Republicans in 1860 had no intention of abolishing slavery where it was already lawful, but they did not want to see it expanded anywhere and wanted it abolished wherever possible.
And again, Why? Not for the reasons most people have been led to believe.
Lincoln was not willing to break what he understood the laws to be, to abolish slavery.
But he did. No due process and continued confiscation after the war. Also the intimidation of states to force them to vote for an amendment they did not want.
DiogenesLamp: "Why? No one will like a truthful answer to this question."
The moral roots of abolitionism are the Bible and Northern churches.
The economic roots of abolitionism came from free-men not wanting to compete against "free" slave labor.
The political roots of abolitionism came from Northern politicians not wishing to grant Southern slavocrats more unbalanced political power than they already enjoyed, via the Constitution's 3/5 rule.
Those seem perfectly reasonable to me, so your problem with them is what, exactly?
DiogenesLamp: "But he did. No due process and continued confiscation after the war. Also the intimidation of states to force them to vote for an amendment they did not want."
First, Congress defines "due process" and second, there was no intimidation -- none, zero, nada intimidation -- of lawful voting Southern US citizens after the Civil War.
Of course, that category did not include those who had declared themselves to be non-citizens!