The secret to JB's success, as time went on, was his avoidance of conflict. The mid-19th century was an explosive, divisive time, and Old Buck got through it without much baggage. It was great for his career, but it inspires no respect.
He served alongside what Klein calls a cache of Parliamentary giants - not only future presidents like Van Buren, Polk, Filmore, and Pierce, but lifelong statesmen like Webster, Clay, Calhoun, Douglas, Benton, and Crawford. These men got their hands dirty. They loved speechifying. They defended their political interests fiercely. "Best not to say anything," said Buchanan from the sidelines. "Best to see how this all plays out." He wasn't uninterested, or unintelligent, but he was gun-shy. A few early misunderstandings with Andrew Jackson that threatened his political standing made him wary to enter the fray.
During the four gargantuan Congressional battles that led up to the Civil War - the Missouri Compromise, the nullification crisis, the Compromise of 1850, and the Kansas-Nebraska bill - Buchanan was elsewhere. Either not in office or abroad. He was, of course, Secretary of State during the Mexican-American war, but that hung solely around Polk's neck.
As the 1850s started getting real crazy - John Brown, Kansas-Nebraska, the Fugitive Slave Act's big comeback - Buchanan was in England negotiating fishing rights. He returned to America in 1856, just in time for the election, and everybody said, hey, nobody hates this guy!
So they gave the reins of the country, in its darkest hour, to a chronically passive man.