Slavery started in the North American colonies for two main reasons. First, there was a need for unfree labor. Second, other sources of unfree labor dried up and/or became less acceptable to the colonial elites.
First, slavery started in the colonies because the rich planters in the colonies needed workers who were not free. Free workers were generally not willing to do the hard work in poor conditions that the planters needed. Free workers were not willing to do the hard work for others because they could easily go out and get land for themselves. Free workers strongly preferred working for themselves rather than working for someone else. Therefore, the rich planters needed people to work for them who would not be free to leave and work for themselves.
But why did they choose slavery? Why not use indentured servants like they did at first? There are two reasons for this. First, economic conditions improved in England. This made it so that fewer people wanted to indenture themselves, making themselves, in essence, slaves for a number of years. Second, the colonial elites worried about having too many ex-indentured servants in their colonies. They worried that these people would rise up against the elites if there got to be enough of them. Therefore, they wanted workers who would never be free. The obvious solution to this problem was to enslave people for life.
Thus, slavery began in the colonies because there was a need for unfree labor that would remain unfree in perpetuity.