They existed, just there weren't many of them. About 2% of the total number of slaves brought from Africa to the Americas came across the Atlantic on North American ships.

The Atlantic Slave Trade predated the discovery of the Americas, as Portuguese voyages to the western coast of Africa were bringing a few slaves back to Europe by the mid-fifteenth century, who mostly ended up as domestic servants. But the colonization of the Americas completely changed the slave trade- intensive plantation agriculture of sugar, coffee, cotton etc. demanded incredible amounts of labor, and resulted in a high rate of mortality among enslaved populations, especially in tropical regions. And so the vast majority of slaves taken from Africa between 1500 to 1800 went to either Brazil or the Caribbean, which were the best areas for such plantation agriculture.

Nearly 40% of all slaves who went to the Americas ended up in Brazil, and unsurprisingly the Portuguese, who had been involved in the Atlantic slave trade since the beginning, were responsible for shipping nearly 40% of all slaves across the Atlantic. The British, who dominated maritime trade (especially in the 18th century) and who wanted to fuel the slave economies of their Caribbean colonies, were responsible for about a third. The French, Dutch, and Spanish were responsible for nearly all of the rest of the slave trade, with about 13%, 6%, and 3% of the total, respectively.

By the time that the Thirteen Colonies were up and running to the extent that local American shipping businesses could get involved in the slave trade (c. 1725), the European powers had all been at it for centuries. But there was a market for slaves, especially in the southern colonies, and so some North American merchants got involved- they were just really late to the game. Then in 1807, the United States (and coincidentally, the UK) banned any further importation of slaves, cutting the few North American merchants out of the trade.

Stats from Cambridge World History of Slavery, volume 3.