If we had some states allowing it and other states not to allowing it, the number would certainly decrease. Not everyone will travel 1000 miles to murder a baby.
More importantly, I think, couldn't states that allowed abortion be challenged, point by point, in a federal court on Constitutional grounds? Isn't that the system of separate powers, representative legislation, Constitutional law and decentralized government our founders had in mind?
With fifty states making laws on the subject there would be that many more potential Constitutional challenges. Each time SCOTUS ruled on one aspect of law in relation to abortion regarding one state's laws the other 49 would have to come into compliance with that ruling.