That’s what happens when you kill it’s driving force and replace him with a sympathizer.
Let’s not pretend that Lincoln being there would have magically resolved it all. Let’s also not pretend he’d have cared to implement it to the level anyone might think.
We mythologize him, but he was ultimately a politician making calculated decisions for his career. That’s why he wasted effort seeking more electoral votes by getting Nevada made into a state. That’s why he chose Johnson to ensure he’d have some favorability with the south.
He did the right thing because it was politically convenient.
Lincoln wouldn’t have magically resolved everything, as Lincoln was still from the moderate wing of the Republican Party (even if he became more radical as the war wore on), but it’s hard to imagine a worse successor than Andrew Johnson, who wasn’t even part of the big tent antislavery party. Traitorous fuck, no better than the copperheads.