First, rating Presidents may be a popular thing but it really isn't useful for understanding their history. Their situations and challenges were quite different: Lincoln had a real Civil War to think about, while Cleveland could focus on things like the tariff. Buchanan was a northerner biased towards the slave-holding south: Johnson was a slave-owning southerner who was against Secession. With these similarities, perhaps if their roles had been switched they would have acted the same and the ratings follow them. Attitudes and norms in the US have also greatly changed with time and vary from place to place: Warren Harding's possible adultery was a much more serious charge for him in 1925 than racism was against Wilson. Some Texans love Polk for , as they would put it, having the United States join Texas.

But, anyway, a short answer would be that Buchanan rates worse because he had a greater effect. The North and South had found it more and more difficult to continue to compromise on the issue of slavery, and southern secession was more and more difficult to avoid. Buchanan favored the south: he meddled with the Supreme Court to influence the Dredd Scott decision, which hastened the onset of the War by allowing slavery everywhere. And he did almost nothing once the southern states began to secede. You could argue that even the best diplomat could only have delayed the War by a few years, but you can't escape the fact that Buchanan could have had significant influence because there was a divided Congress , yet did nothing much...and what he did was, really, on the side of the south: and that was the wrong side to take.

Unlike Buchanan, early in his term Johnson faced a unified Congress. It was determined to coerce the South, and though he could somewhat delay Reconstruction- vetoing the act creating the Freedman's Bureau, for example- facing a complete Republican opposition, he had less effect. When Grant followed him, Reconstruction happened anyway- for a time.