Nations that were mainly owned by whom at the time?
Not to mention Protugal, having made inroads earlier than other European colonial powers, actually did capture most of the slaves they utilized or sold themselves, whereas others who came later and lacked access to widespread infrastructure and an operating system as well as men to run them, such as the Brits or Germans, ended up often losing too many of their own people to make the effort cost effective (or even a net gain), so opted to rely on locals to buy from, then selling those abroad at huge markups.
London, that capital of finance, became that through commodity markets, banking and insurance, etc, centered around and profiting off the slave trade. Literally.