From the slaveowner's point of view, Indians would have seemed like a good idea because they were readily available. If you want slaves and you have a group of people already living near you that is weaker than you , they would seem like a good source of slaves.
However, it soon became clear that this would not work. Most importantly, it was too easy for the Indians to escape. In addition, enslaving them might well bring about retaliatory attacks on white settlements.
Slaveowners soon turned to Africans as slaves. Part of the reason for this was that many Africans (unlike Native Americans) were used to working in large scale agriculture. But the main advantage of using African slaves was that they could not easily run away. They were, the slaveowners reasoned, easily identified as slaves by the color of their skin (unlike white indentured servants). Unlike Indians, they had no way of running away to rejoin their own people. This made them ideal as slaves.