Communism

Slaves don’t earn wages to buy products. The slave-owners have to spend funds to feed, house them etc.

But it was really more about creating consumer markets. There is no capitalism without vast consumer markets for mass produced goods.

Mercantile slavery produced less, more artisanal goods, for a very small class of people (aristocracy and nascent bourgeoisie).

And it wasn’t possible to expand the consumer markets without creating a new class of people who had an income to spend on commodities.

This is extremely simplified, but it’s the main interpretation for the end of slavery. Like, when we study the Industrial Revolution, the British empire and the end of slavery in school, it’s always under that lens, of creating new consumer markets.

But just to make it clear, slavery is still lucrative, to this day. Which is why we have more slaves in absolute numbers now than at any other point in history. But it can’t be the main relation of production, because capitalism depends on mass consumption by masses of people. So slavery can only ever exist as a marginal system.