Their primary industry was making ships and using ships, so they were pretty much constrained to carrying some kind of cargo, and of course their cargo was human beings.
Had they decided they could work hard and prosper on their own, then the need for slaves would have been greatly reduced.
You should read George Washington's memoirs of his usage of slaves. He lamented that it was difficult to find tasks for them to do that were worth the cost to him of keeping them. This was the reality in 1790, not just for him, but for most of them. Eli Whitney changed all that.
Their primary industry was making ships and using ships, so they were pretty much constrained to carrying some kind of cargo
?