Mmm. Yes, how horrible I am wanting to lock in abortion rights as a federal law which would override the states that have banned it.
Why wouldn't the Supreme Court just nullify that law the same way the court did to RvW?
To nullify that law, they would have to demonstrate that
the law itself is unconstitutional.
It's a different standard. It's not a perfect parallel by any means, but you could think of it like the difference between a criminal and a civil trial. In a civil trial, the prosecution only has to demonstrate a "preponderance of evidence", whereas in a criminal trial, they have to demonstrate "beyond a reasonable doubt". The bar is much higher.
For the original RvW, it was based on a judicial interpretation that prohibiting abortion was a violation of privacy. The constitution doesn't actually explicitly guarantee a right to privacy - that in itself is an interpretive view. The 14th says the state can't deprive citizens of life, liberty, or property without due process of law. That has been interpreted several times to imply that we have a right to privacy in many aspects. So RvW was kind of an interpretation (that abortion is a matter of privacy) on top of an interpretation (that privacy is guaranteed by the constitution).
If it's passed as an actual federal law, then it stands on it's own. To be overturned, the courts would need to show that it's an illegal law - and I don't think there's anything in the constitution that would support that. It's still possible, sure... but much more difficult to do.