As always, the pro-baby-buyers fail to consider that life is complicated for women.
The Pregnancy Choices board is full of women who are considering ending their pregnancies for various reasons. Women who've realised that their husbands are abusing them and don't want to birth a child who will be used as another tool with which to abuse. Women whose husbands have left them or who've realised that their husband is cheating and don't want to raise a child alone.
If Alice and Bob commission Carol to bear their baby and Alice realises at 12 weeks that Bob has cheated on her, she has no right whatsoever to instruct Carol to abort so that Alice can leave Bob unencumbered by a shared child and have a fresh start. Carol would be left literally holding the baby, a baby that she thought she would be handing over. Any law that allowed Alice to force Carol to abort would be in violation of Carol's human rights.
Some 40% of pregnancies have complications. Some women abort because of these complications. If Alice is paying Carol for a baby, which is what happens with overseas surrogacy, that discourages Carol from putting her own health first if an abortion would be the best option for her. Even in the altruistic case, Carol knows that she risks her friendship with Alice by aborting. If Alice and Carol are sisters, an abortion even on medical grounds would probably result in estrangement.
Women are not walking incubators, but people, with full human rights. Babies are not commodities, but people, with full human rights from the moment that they are born. There is no way to have surrogacy that respects the full human rights of the baby and the surrogate mother.