The nation had compromised many times on the slavery question before 1860. Why was the nation unable to compromise again in 1860-61 and how did...

The leader of the Compromise of 1820 and the Compromise of 1850, Henry Clay, died.  His voice of moderation was sorely missed in Congress in 1860.  There was finally a political party dedicated to abolitionists, the Republicans, and for the first time they put a candidate in the White House.  While Lincoln was a moderate Republican who only blocked the expansion of slavery, many slaveholders in the South feared that if slave states did not expand with respect to the growth of free states in the West, then at some future time the Senate could vote to end slavery.  By 1860 there was also more money in slavery than ever before, as European and Northeastern textile mills needed Southern cotton.  It would have been impossible for the federal government to issue fair compensation for all the slaves in the South in order to buy them and emancipate them, and slaveholders thought that the federal government might outlaw slavery instead in order to get around this moral predicament.  

Feelings about slavery were also never stronger in this country than during the 1850s.  Uncle Tom's Cabin, a book by Harriet Beecher Stowe, often painted slaveowners as greedy or callous and many people wanted slavery ended on humanitarian grounds.  John Brown became a martyr after he was hung in 1859 for trying to start a slave revolt.  The civil war between abolitionists and slaveowners in Kansas and Missouri proved that popular sovereignty could not work and that the nation needed a federal policy on slavery.  Also, the pro-slavery decision in the Dred Scot case made many in government think that the Compromises that saved the nation may not even be constitutional.