sirbobby98121
Two possibilities - it was an established policy and there was no feasible technological replacement to a manpower shortage.
Aspen
I think it was a compromise when they were trying to prosecute a war of independence which required all the colonies to unify and stand as one. There was already some recognition that there was inconsistency and potential hostility between northern states and southern slave states. But they ignored these differences in order to win a war.
socialistpb
Because many of them supported slavery.
Anonymous
Founding fathers like Jefferson and Washington, who owned slaves, knew about the conflict but didn't know how to resolve it. Both figured it would resolve itself as tobacco became less grown by how it ate the soil. But, the invention of the cotton gin wrote history. Slave labor became more profitable than ever.
There are plenty of books to read on this. My contribution is intentionally limited.
Christian
Why did almost every other country have slaves? Profits. Don't forget that we're all still animals.