Not to sound cold but it is all about supply/demand (hence also the longer waiting period ). Our agency states that the extra fees are because 1) advertisement to persuade women considering adoption to come to them - this is probably the biggest reason why it is so costly, 2) some of the fees may go towards living expenses for the BM - white women tend to have higher expense needs than women of color.
Although we did not choose to go down the path of adopting a baby with african american heritage because of the cost - it certainly was a factor. In the case of children with ANY AA heritage the costs are generally 50-75% less and the wait periods are typically 75% shorter than that of a white newborn.
It is easier to charge more and there is a need for more money to be spent advertising (especially if your agency works with all states) when an agency may have at any given time hundreds of families waiting 'in line' for a white baby - they have do everything they can to increase the number of BMs that come there way. Unlike babies with AA heritage there are more BM than adoptive couples - so they reduce the fees greatly and offer government subsidies to encourage people to adopt non-white children.