Why were ancient civilizations based on slavery?

Because at the head of each such ancient state was a despotic ruler, where society itself was built on a slave system, where one was subordinate to the other. At that time, there were several ways to become a slave: captivity or debt slavery. In fact, the slave system, as such, was able to survive until the 19th century. Especially if you look at the United States, where Africans were slaves for a long time, and even the Russian Empire, where the bulk of the population was dependent.