Most of this is for sitting on his thumbs while the country split in two. Lincoln is largely regarded as the "best" president mostly based on his resolve in preserving the union. Buchanan gets the opposite treatment.

He had an interesting philosophy. He thought the southern states didn't have a right to secede, but he also believed in limited presidential power. So he didn't think he had any authority to stop them. Instead he sat in the White House and waited for someone else to come up with a solution. That lack of decisive action cost the country dearly, as he may have been able to sooth southern fears or nip the insurrection in the bid. Instead the secessionists had valuable time to rally support and organize themselves.

But even without, you know, that whole Civil War thing, it's not like he had an otherwise successful presidency.

He deeply interfered with the Supreme Court. He didn't much care what the rulings were, but he wanted large majorities supported by justices from both sections of the country. This led him to pressure Pennsylvanian Justice Robert Grier to support the southern justices in the name of court unity. His interference incidentally helped the Court turn in one of its worst ranked performances in history. It also had the opposite effect of what Buchanan wanted. The Taney court didn't create a national consensus. Instead it deepened the sectional divide as the North saw it as a Southern takeover.

On top of that, he directly oversaw one of the worse economic periods in US history, the Panic of 1857. He actually got his signature bill, the Tariff of 1857 passed the day before he officially took office. That deeply hurt American exports and widespread "layoffs" were almost immediate. With a major downturn in production, rail use fell. By mid-summer railroads started declaring bankruptcy and lines were closed, resulting in a run on banks. All through this Buchanan explicitly blamed individual speculators who "deserved the gambler's fate." His cold comfort exacerbated the problem. Very few men can claim sole responsibility for starting a Great Depression. Buchanan comes about as close as possible.

His use of military was really weird to boot. The Utah (Mormon) war, interventions in Guatemala, Nicaragua, and Paraguay, and the "Pig War;" Buchanan liked to use force (sometimes extralegally) in times and places where it wasn't needed. And yet seemed really unwilling to use it in places like Bleeding Kansas where it might have done some good. In fact the whole Kansas statehood situation was just a badly mismanaged disaster, ratcheting up tensions toward the Civil War.

And those are just some of his biggest lowlights. Just about every decision he made seems to have been the wrong one.

It's odd because he had been a successful legislator. He also had done ok as Secretary of State. So his terrible turn as president really wasn't expected.