Why does what is "right" and what is "wrong" change?

How is it possible that what is "right" and what is "wrong" has changed over time? Take slavery for instance. In the bible its no big deal to have a slave servant or to. And also, having sex with young girls. Many women were wives in their early teens a lot of times back then. But now, both of these things are considered "wrong" So, what makes it right back then but wrong now? Is it just whatever the majority of the population believes? In that case Christianity is "wrong" because most of the worlds population is not Christian, and also Jesus was NOT the lord because most people did not believe that either at the time of his existence on this earth. So, what is the consistent reason here?