Free market economies routinely bury slave based ones. If slavery was more profitable, it would be the other way around.

Modern human trafficking is prevalent in industries that are themselves illegal, so there isn't free market competition.

The problem with slave labor is considerable expense is necessary to guard them, and the slaves not only do as little as possible but they will actively sabotage the work.

We, collectively as a species, managed to find a path in which slavery was no longer necessary - for the first time in human history.

Yes, the cost was huge - that's why stupid things like the Fugitive Slave Law and the Dred Scott Decision happened, leading to the failure of Whiggish Moderation and, well, you know the rest...