Why were blacks slaves in the 19th century?

In the 19th century, there were independent states on almost all continents, and only in most of Africa there were none. In addition, Africa, despite its territorial proximity to Europe, was the last of all continents to be studied. There have always been slaves, a slave is a person who has no right to choose, who works only for food, is subject to beating and is completely dependent on the owner.
In the 19th century, neither Europeans nor Asians could be slaves, since they had rights. The Indians in America also did not become slaves, since they felt themselves masters in their land and did not obey the settlers. In the 19th century, only Africans were slaves, since they were easier to transport – this was facilitated by the developed sea routes. The skin color of Africans was different from the skin color of the inhabitants of other continents, so they were called not people, but animals, and they were treated differently from ordinary people.