In the Colonial Period, Europe had some ideas about the enlightenment that made slaves unpleasant to their sympathies. For that reason, despite the English having slaves in their colonies, you could become a free man by visiting Britain's shores (there were some interesting court cases related to this).
More important to this era was poverty, which was a big issue in European cities. So there was no shortage of cheap labour; even with the dying out of serfdom. Serfdom lost popularity in Western Europe around the 1300s (though cases remained until the 1500s). The practice was continued in Eastern Europe for a lot longer, to try and fill the gap in the agricultural market that arose with the reduction of serfs. After the Napoleonic wars, serfdom mostly came to an end in Europe, save Russia.
The place where a demand for cheap and harsh labour was in force, was in the colonies with their plantations. Recruiting people from the European homeland to farm this would've been difficult to convince them and expensive to pay them, so slaves were the economical way out.