The Bible (and the God of the Bible) often get tomatoes thrown at them for seeming to legitimize slavery.
Never mind the fact that Christians have been at the forefront of the abolition of slavery throughout history, it is true that the Bible does not go out of it’s way to condemn slavery at every possible turn.
Why? Whole books have been written on this topic, but here I think Fearsome at the BHT gives the best simple answer I’ve ever heard:
I think that if anyone had come up to Jesus and asked him about slavery, he would have said something along the lines, “Moses wrote the laws concerning slaves because your hearts were hard.” So in other words, the slavery laws in the OT are like the divorce laws–they are the regulation of a common evil in order to mitigate its effects.