Because that child could still be born and live a life. For all you know the aborted child could invent a cure for cancer. Just because the mother can't care for it doesn't mean that it can't be put up for adoption.
While TBR and reece have certainly debunked emporer1's argument, the point still stands. What's wrong with adoption? You don't have to take care of the child, the child still lives, and someone somewhere who physically CAN'T have children have the opportunity to find a child anyways. As far as I can see, there is no downside to making abortion illegal and just supplementing adoption instead.