According to certain Marxist authors (Mostly Georgi Plekhanov (and some others), but I'm afraid I cannot quote because the text is not in electronic form and not in English.), slavery in US and latter in South America did not disappear because of enlightenment and abolitionist movements ("spiritual" reasons). Instead, there were purely materialistic (technological and economic) causes that made slavery non-profitable and obsolete. This economic pressure resulted not only in abolitionist movement, but also created rift in society, big enough to cause US civil war.
My question is are there any reliable economic data about profitability of slave plantations on one side, and plantations with payed labor on other side, immediately before and after US civil war. Slave plantations certainly had advantage of unpaid labor, but also had to feed, house and clothe slaves. Also, slaves had no incentive to work harder except for fear of physical punishment. On the other hand free paid workers had to earn money to take care of themselves and their families, so they were better motivated to work harder.
EDIT Upon request, I will add some quotes about Marxists perspective on US civil war . This is recurring theme in Marxist literature, it basically says that slavery was obsolete, and that Southern slave owning ruling class tried to protect their vanning influence by secession. On the other hand Northern capitalist ruling class wanted to convert whole US territory to their economic model.
From Marx himself :
The cultivation of the Southern export articles, cotton, tobacco, sugar, etc., carried on by slaves, is only renumerative as long as it is conducted with large gangs of slaves, on a mass scale and on wide expanses of a naturally fertile soil, which requires only simple labor. Intensive cultivation, which depends less on fertility of the soil than on investment of capital, intelligence and energy of labor, is contrary to the nature of slavery.
Also this :
[T]he number of actual slaveholders in the South of the Union does not amount to more than 300,000, a narrow oligarchy that is confronted with many millions of so-called poor whites, whose numbers have been constantly growing through concentration of landed property and whose condition is only to be compared with that of the Roman plebeians in the period of Rome’s extreme decline. Only by acquisition and the prospect of acquisition of new Territories, as well as by filibustering expeditions [i.e. conquests of other lands, such as in Central America—ISR], is it possible to square the interests of these “poor whites” with those of the slaveholders, to give their restless thirst for action a harmless direction and to tame them with the prospect of one day becoming slaveholders themselves.
A strict confinement of slavery within its old terrain, therefore, was bound according to economic law to lead to its gradual extinction, in the political sphere to annihilate the hegemony that the slave states exercised through the Senate, and finally to expose the slaveholding oligarchy within its own states to threatening perils from the “poor whites.” In accordance with the principle that any further extension of slave Territories was to be prohibited by law, the Republicans therefore attacked the rule of the slaveholders at its root. The Republican election victory was accordingly bound to lead to open struggle between North and South. And this election victory, as already mentioned, was itself conditioned by the split in the Democratic camp.
But Marx was also critical about motivations of Northern capitalists
As a slave, the worker has exchange value, a value; as a free wage-worker he has no value; it is rather his power of disposing of his labor, effected by exchange with him which has value. It is not he who stands towards the capitalist as exchange value, but the capitalist towards him. His valuelessness and devaluation is the presupposition of capital precondition of free labor in general.27 In slave-labor, even that part of the working-day in which the slave is only replacing the value of his own means of existence, in which, therefore, in fact, he works for himself alone, appears as labor for his master. All the slave's labor appears as unpaid labor. In wage-labor, on the contrary, even surplus labor, or unpaid labor, appears as paid. There the property-relation conceals the labor of the slave for himself; here the money-relation conceals the unrequited labor of the wage-laborer.28
Finally, even today socialists and left-leaning authors then to hold US civil war as a conflict between ruling classes primarily concerned about profit, not ethics:
From a Marxist perspective, here is the true cause of the main stage of the American Civil War. The Union, or the North, had developed an economy based on wage slavery; whereas, the Confederacy, or the South, had retained chattel slavery. These two economic system were both subdivisions of Capitalist economics, or the private ownership of the means of production. Theoretically, it is impossible for these two subsystems to coexist peacefully in any economy; so, an ultimate conflict to determine the future of capitalist production was inevitable. The remainder of the circumstances are naturally absorbed by this reality.