It was unconstitutional because the SCOTUS made a law from the bench.
And the wienies in the legislative branch sat back and let the judicial branch usurp a power only vested to them by the constitution.
Not surprising since ten years earlier the legislative branch let the executive usurp the power to declare war with the Gulf of Tonkin resolution.
The legislative branch has become WORTHLESS.
Real power is NO longer directly transferred at elections.
It is transferred at the confirmation hearings in the senate.
So-called Democracy doesn’t get any more down and dirty than the Kavanaugh confirmation hearing.
It understandable why the Marxists want to “pack” the court.
That is also a good reason why it’s unconstitutional. But the God-given right to life is the reason it would have been unconstitutional even if SCOTUS had the authority to make law.