Because he was. He believed in white man superiority (research his speeches leading up to the war), Emancipation Proclamation didn't happen until mid-way through the civil war when he needed more money and war support (it's crazy, but people actually hate war) and he was a hardcore Federalist/Statist who believed in a singular power structure.

Lincoln's ideals were very much against our founding documents and the ideals that this country was supposed to be built upon. History remembers him as a great liberator. In reality he was a tyrant.