I tend to think that, throughout time, people have been notoriously lazy. Those with power, therefore, could force people weaker than them to do the grueling labor needing to be completed. In history, people tended to own large amounts of land and needed help maintaining the land (crops, building, etc.). Given that there have been multiple battles in order to prove superiority, one thing the winner could do was put the losers to work.
If one were to look at the Egyptian and Roman periods in history, many slaves were used to build the monuments erected in order to honor the ones who enslaved the masses. People generally had no other choice but to submit given their inability to protect themselves and stand against the oppression.
Given that the use of slaves was so successful, slave labor continued. People in power could, most likely, not see paying for labor when they could simply take over other societies and enslave them for free labor. Also, some simply saw that their superiority gave them the right to oppress those who they saw as weaker.
Some could also refer to the evidence that slaves were used in the Bible. (Meaning that if they were used during this period, then it could not be seen as wrong--very assumptive on my part, and it is simply based upon my own interpretation of the history of slavery).
I would argue that it has been tolerated by so many societies for two reasons.
First, it has been economically useful in many times and places. It has been a way of getting people to do hard jobs that they might not be willing to do voluntarily. It has provided larger workforces to places that might not have had enough population.
Second, the people of the societies have been able to see the slaves as somehow worthy of being enslaved. This has meant that slavery has not seemed inhuman. The slaves have been those who have been conquered or they have been from a different race. In both cases, those who enslaved them could rationalize the slavery by saying that the enslaved were inferior in some way.
Thus, slavery has been tolerated because it has been useful and because societies have been able to rationalize it in ways that make it seem less inhumane to them.
Well, to start is just human nature for people to feel the need to feel superior to other men. Also, its cheap labor and saves a lot of money. Up until the last century agriculture was the biggest bussiness economically, and it still is in many countries, so they wanted to find a way to get as much product for a lower price. As for ancient times, civilizations such as the Romans and Greeks used slaves as a form of punishment for people who are prisoners of war or are in debt to their master. In short, slaves were a part of society for their cheap labor, punishment for crimes and debts, and so that the rich could feel superior to the poor.