In this period, if you had a worker or a slave who couldn't ride a horse, couldn't care for a horse, was scared of horses, i mean, that would make them a lot less valuable. In africa, they had large scale agra business, and they grew all kinds of stuff. They grew things in africa that the europeans wanted to grow in the new world, but didn't know how to grow very well in the new World. So the africans showed them how. Hum all of a sudden, not too hard to see why one african slave might be worth more than multiple indian slaves.