Slavery was less efficient than a market economy and a free labor pool. Consider the Irish who built the Erie Canal. They were paid mostly in rum and they died off so quickly the bosses didn't care because a fresh crop of Irish workers walked off the boat the following week.

Slaves had to be fed and housed and nominally cared for. After all, they cost money and you didn't want to throw away your investment.

The slavery economy, which is what the South fought to preserve in the American Civil War (let's face it: those who like to hide behind "states rights", it all comes back to "states rights to buy and have slaves"), was a last gasp to try to preserve what was left of feudalism. It was doomed before it began.

Please don't misconstrue this as being "pro slavery". Treating humans as property is an abomination. Even the Romans believed this and enshrined it into their legal codes even though they kept slaves.