That is the title of one of the current topics at watchtower.org. While the slave trade may not have been condoned by God, article doesn't really answer the question as to why God did not condemn slavery outright. It seems to me that the Bible takes a neutral stand towards slavery, accepting it as a social fact. The New Testament, in particular tells slaves to be obedient to their masters but that masters should be kind to their slaves (Eph. 6:5-9; Col. 3:22-4:1; 1 Tim. 6:1; 1 Pet. 2:18)
There is no attack on the institution of slavery directly. At best, the principle of inequality is addressed. (1 Cor. 7:21-23) There is no distinction between slave and free in Jesus (1 Cor. 12:13; Gal. 3:28; Col. 3:11). If one adheres to Christian love and unity, then how can one regard another man as property?
In this country, the only effective destroyer of slavery was not the Bible or the churches, but the Civil War and man's laws. The churches got on the bandwagon after the fact, with perhaps one exception, the Quakers.
We also must not forget that slavery still exists in some countries. And it probably won't be the churches or the Bible that will bring its demise there, either.