That's a tough one, and there are two distinct answers. The facile answer is that it's been too controversial to ever ram through Congress in some form. Almost no other issue sparks this amount of animosity on both sides. The other answer is that we have a very strange system of government whereby Congress can only enact certain types of laws, mostly to do with finance and the military. Many of the classic criminal laws here (think murder) are state laws. When the feds want something done they pass a law that says something like, "States must do X or else we won't give them any money." Congress passing a law that says "abortions are legal" probably isn't possible. But on the other hand it should really need to be possible, because historically rights have only ever been expanded in this country. They haven't really ever been taken away (except in some really rare and transient cases--during WWII for example), especially in cases where the public so overwhelmingly is on the side of the right that's being debated. So for the average American abortion has been settled for 50 years. The republicans used it as a wedge issue to get votes in the 80s, and since then it's taken on a life of its own (no pun intended) and some of them believe their own bullshit at this point. Public opinion is so firmly against the Court on this one, that I fear it's going to get out of hand one way or another.