Because of the The Atlantic slave trade.
European maritime and commercial expansion from the fifteenth century dramatically changed the history of mankind by bringing together three continents: Europe, Africa and America (we could consider Asia as well, but this is another story). In search of enrichment, the Europeans (the Portuguese were pioneers) organized a whole political, economic and military apparatus that guaranteed them control over Africans and Americans. Thus came what we call the colonial system, which lasted from the sixteenth to the nineteenth century