Early on there were attempts, but there were a few key factors that made Native-American slavery never take on in the English Colonies/United States.

First among these was disease. European diseases had been introduced to many parts of Africa earlier on as part of the extensive trade network between Europe/Asia/Africa. African mortality was thus lower than Native-Americans who had never been exposed.

To give an idea of how devastating this was, between 1616 and 1619 the Native population of coastal Massachusetts is estimated to have dropped from approximately 23,000 to a few hundred. This trend repeated itself heavily across the colonies.

Next, it was near impossible to keep them from running away. If a Native American decided to flee your field and head into the woods, they could very likely communicate with others in the area and meld into another community. Chances of recovery were slim.

African slaves greatly changed this. They generally would not know the language, could not hide in Native American populations, and were unfamiliar with the local terrain outside of their specific areas.

Finally, and a point worth mentioning, is that Native Americans would be taken as slaves from time to time. In the case of the ship Desire, the first recording slaves being brought to New England, some of these Native American slaves were shipped to the West Indies and traded for African Slaves.

Finally there was the economic impact. In order to enslave Native Americans would require conflict on a massive scale that would quickly drive up the cost of colonial defense. On the other hand, African slaves were a market commodity that was already established, and cost less for upkeep in the long term.