Regulate this away:
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.
By contrast, if Alice was pregnant, she would be in control of her abortion decision and could take that fresh start.
If the law forced Alice to take the baby that Carol refused to abort, what kind of life would that baby have?
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.
By contrast, if Alice is pregnant with her own child, it's up to her to decide how far she is willing to risk her health for her baby. She doesn't fear the loss of surrogacy income. There can be no contract that could financially penalise her for prioritising her own health.