It's important to note that free laborers would not work the plantations. They wanted to go off and work their own land (which they could get relatively cheaply). This meant that the workers had to be forced into working. That is where slavery and indentures servants come in. On indentured servants, I'd add that they were white and generally English and therefore they felt that they had various rights that had to be respected -- they were harder to push around.
Because it was the most economical and practical answer to the Southern colonies' need for a labor force. With the kinds of cash crops that were grown on plantations there - tobacco, rice and indigo mostly - it required a labor intensive process of almost daily agricultural work and care. Indentured servants were OK for a while, but they were temporary, usually finishing their contracts within 5 to 7 years, after which you had to rent and train another one. Native Americans simply escaped and went home and could not be captured in the numbers needed.
With slaves, not only did the owners get a permanent worker, but they also were legally entitled to all of their offspring. It was a very brutal and ugly institution, but in the 17th through the 19th centuries, it was the most economically viable solution to southern plantation owners' labor needs.
I believe that the posts # 2 and 3 above very much reflect the logic slave owners would have advanced to justify the use of slavery. It should not be noted that this is no justification for slavery. The slave owners in America and in many other places and in many periods used slavery to force people to work for them because that was the only way they could think of making life easy for themselves. However this does not mean that slavery was ever the right way to get best output from men.
In modern times the practice of slavery has slowly disappeared not because people have become kinder but because they have found better alternatives to slavery. In this connection it is imporant to note that in late 19th and early 20th century the economic condition of labor force in industry was much worse than that of the slaves in the USA prior to abolition of slavery. But with improved understanding of human nature brought about social scientist and management pioneers, the industrialists learnt that they can get the best out of the labor force not by forcing them to life of abject poverty and premature death, but by treating them well.