Why did europeans use one-crop economies in their colonies?

I believe that one-crop plantation economies shaped the development of the southern colonies more than slavery or Indian relations. I believe this because during the seventeenth century, there were still many indentured servants to do work in the southern colonies. Many indian problems had been solved or not as pressing during this time, and with the help of indentured servents and laborers, southern farmers were able to experiment to find crops that grew well in this new climate and land. Thus, one crop plantations growing tobacco, sugar, and cotton become empires in the south, in increasing high demand as the century ends, and thus slavery became a popular form labor.