Catering for the whims of the new parents as opposed to the best interests of the child, which is not to be conceived as a commercial commodity. Commercial operations impregnate cows, and then take the calves away from the mothers so as to use the mothers milk for their own needs, and use the calves for meat.
Giving away a child whose mother is unable to support the child, and/or the child was conceived when it should not have been (due to rape, incest, mother being too young..) is vastly different to a pre-planned conception, using another woman as the incubator, and then taking the child away from it's mother (ie in whose womb it developed). All that is for the benefit of the parties involved, the future parents and the surrogate, and not for the benefit of the child.