Some people think that no abortions are morally acceptable, and some think that most all abortions are morally acceptable. But many people think that some are, and some are not, acceptable, and this likely represents the majority of people.
If we conclude that some abortions are moral and others are not, then it may not be feasible to determine judicially which are which in time to decide whether to abort.
Take, for example the case of rape, the most popular abortion justification. The justice system sometimes takes years to determine whether a woman was raped or not, and he said she said scenarios abound. But pregnancy takes nine months. So we are stuck with putting trimester restrictions on abortions, outlawing them, or pretty much allowing all of them.
All of which suggests that we all too often get our morality from a law book, and that this is a mistake.