I have a theory. We are more moral today because it costs us less/benefits us more than it did in the past. In fact, I would say our sense of morality is always directed by a cost benefit analysis.
Most people today regard slavery as an immoral institution, but for thousands of years, the majority of human history, slavery was quite common. Slavery was not unique to any one nation or group of people and it would be hard to make the case that when slavery was prevalent, most people deemed it immoral. But today, times have changed and slavery is seen as an irredeemable sin. Slavery is this nation's most visible scar and it's unthinkable that any moral person could tolerate its existence.
I personally regard slavery as sinful, but I doubt that my morality came about due to divine intervention. In my opinion, it's unlikely that I, my ancestors, and the rest of humanity have evolved significantly in the last 300 hundred years. Natural selection works, but not quite that fast. It's not like a switch turns on, a light bulb appears, and we say, "hey, slavery is wrong let's stop it!" That sort of absurd arbitrariness works when you turn 18, just not in the course of human history.
The reality is that we have not changed internally, but something external has caused us to modify our behaviors. Something in our environment has changed the calculus regarding the benefits and cost of slavery. Let's think for a moment. Despite notions to the contrary, having slaves is not free. You must feed, clothe, and watch them. They are your property and your responsibility. The only rational reason for keeping slaves is that their labor is worth more than the cost. At some point, owning slaves becomes a negative trade off and even keeping them for some morbid amusement is not economically justified.
In the end, it's our self interest that inevitably drives us. When the outside environment makes one action too costly, we evolve our behaviors, we evolve our morals, so that the previous action is regarded as unwise, wrong. Slavery, in regards to the individual's self interest, simply doesn't make economic sense. Not only is it immoral, it is stupid, and that makes being moral a whole lot easier. A wise person doesn't tempt fate, he also doesn't test his morality by lowering it's price. Capitalism is the evolutionary mechanism which has funneled our self interest against the evils of slavery. Capitalism makes slavery immoral and that's why all good people should support it.