I couldn't find a lot of information on this. It seems that if America was under British rule while the slave trade was established, it was more of an inherited policy(for lack of a better word) from British rule.
You could say that the US was influenced a lot by England but also look at the different European countries coming over at the same time as the British. They were all picking up Africans and migrating west. It would be considered that the British had more influence because they stayed and a majority of people came from England but the British and the settlers did slave trade with other countries thus the British didn't have all the influence