In the US the invention of the cotton gin reinvigorated slavery, which prior to that point had been declining. In the Antebellum south, slavery was profitable, with average return on slaves of about 8 to 10%. Even after slavery was legally abolished, extremely similar social institutions remained for decades (share cropping + institutionalized racism). It wasn't until the 1940s that cotton picking was mechanized.

The North steamrolled the South economically because manufacturing steamrolls agriculture in terms of productivity, but an apples to apples comparison between farmers shows slave owning plantations were economically more productive than free farmers. After abolition, the South's per capita productivity dropped substantially, and remained 20% lower per capita in 1880 than it had been in 1860.

Obviously slaves were individually much worse off than if they were free, and society as a whole suffered from the suppression of human capital. Had all those people been free for all that time, imagine what could have been accomplished besides growing more cotton. But for the slave-owner, there was never a point where abolition became economically advantageous, hence why slavery was ended at gunpoint.