http://www.chabad.org/library/article_cdo/aid/305549/jewish/Torah-Slavery-and-the-Jews.htmLet's start simple:
Take an agrarian society surrounded by hostile nations. Go in there and forcefully abolish slavery. The result? War, bloodshed, hatred, prejudice, poverty and eventually, a return to slavery until the underlying conditions change. Which is pretty much what happened in the American South when the semi-industrialized North imposed their laws upon the agrarian South. And in Texas when Mexico attempted to abolish slavery among the Anglophones there.
Not a good idea. Better idea: Place humane restrictions upon the institution of indentured servitude. Yes, it's still ugly, but in the meantime, you'll teach people compassion and kindness. Educate. Make workshops. Go white-water rafting together. (Hey, why didn't Abe Lincoln think of white-water rafting?) Eventually, things change and slavery becomes an anachronism for such a society.
Which is pretty much what happened to Jewish society. Note this: At a time when Romans had literally thousands of slaves per citizen, even the wealthiest Jews held very modest numbers of servants. And those servants, the Talmud tells us, were treated better by their masters than foreign kings would treat their own subjects.
Torah teaches us how to run a libertarian society--through education and participation. Elsewhere in the world, emperors and aristocracy knew only how to govern a mass of people through oppression. Look what happened to Rome.
Getting Real Change
So you can see where I'm getting to with the slavery thing. If G d would simply and explicitly declare all the rules, precisely as He wants His world to look and what we need to do about it, the Torah would never become real to us. No matter how much we would do and how good we would be, we would remain aliens to the process.
So, too, with slavery (and there are many other examples): In the beginning, the world starts off as a place where oppressing others is a no-qualms, perfectly acceptable practice. It's not just the practice Torah needs to deal with, it's the attitude. So Torah involves us in arriving at that attitude. To the point that we will say, "Even though the Torah lets us, we don't do things that way."
Which means that we've really learnt something. And now, we can teach it to others. Because those things you're just told, those you cannot teach. You can only teach that which you have discovered on your own.
History bears this out. Historically, it has been the Oral Torah, rather than the Written Torah, that has had the greatest impact on civilization. As much as Rome ruled over Judea, Jewish values deeply transformed Rome. One of the results was the legal privileges eventually granted to slaves and the gradual recognition of the value of human life.