Because it became a political appointment.
They really should just decide whether a law is constitutional or not. If they decide not, the legislature and states can still amend the constitution. A long slow process but obviously it has been done.
But now they are another legislative political body. It is a shame but how can we change it back? When we nominate originalists we are told we are “right wing.” That really is not a “wing” position.
They stopped upholding the Constitution in the 1930s when they helped FDR kick the Constitution to the curb. Now they uphold Arbitrary government where they can make shit up as they go along.
Where in the text of Article III, or anywhere else, do you find that power?
This rot goes back a long way.