Is it a right because it is legal, or is it legal because it is a right?
Neither. It's a
legal right because people want it to be a legal right. We live in a democracy, and if enough people want something, they can get it. They wanted the right to an abortion, so they got it.
If, however, you're looking for what makes abortion a
moral right, it's a little tougher.
If you adhere to absolute morals which dictate abortion is wrong, then it is wrong, and nothing anyone says can change that.
If you adhere to absolute morals which dictate that abortion is okay, then it is okay, and nothing anyone says can change that.
If you adhere to relative morality, it all depends on the time and place in which you are living. In the here and now (US in early 21st century) abortion is legal. It is a legal right, and the people who made it so believed it was a moral right also (indeed, made it so
because it is a moral right). So, relative morality tells us that abortion can be a moral right, and
is a moral right (in the here and now).
I guess it all comes down to who or what dictates your morals as to whether or not it is a moral right.