Indentured servants were people residing in the colonies who would sign a work contract, usually for 6 or so years, in exchange for land or even getting their crimes pardoned. Indentured servants were NOT considered to be property of the contract holder and upon their contract being up, they were able to be an equal part of the community (able to own property and vote). A slave was brought over from Africa (mainly) against their will and had no definite contract. Sometimes slave holders would let the slave buy their way to freedom, but most were slaves for life. If they could "buy" their way out, they received nothing in return for their laboring years and could not own property or vote.
Slavery became the most popular form of labor in the colonies because the slave owners did not have to give the slaves anything in return, could have them indefinitely, and in all honesty just didn't view them as people. It was both a moral issue as well as the fact that the slave owners could just save more money in the end.
