I'm guessing that they turned this down mostly because S.397 could crush the case before it goes to trial. Also, in the absense of an actual trial, they can't say that the law is being improperly applied.
Also, the plaintiffs foolishly argued based solely on the Commerce Clause, when they should have used the prohibition against Bills of Attainder, and the Fifth Ammendment's prohibition against the taking of private property.
The few Justices who interpret the Commerce Clause more narrowly would not have wanted to make a ruling in favor of the gun manufacturers by means of a ruling that distorts the Commerce Clause even further, and the other Justices are simply clueless.