What leads you to believe that? The only "better" case would be one that merely challenged the federal ugly gun ban. This case had everything else. A dispute between the circuits, non-criminal upstanding citizen appellees. On further reflection I'm not sure a pure federal challenge would neccessarily have been better. This way the Justices would have gotten express and affirm federal supremacy, and they always are wont to do that.
I notice that this case was the lowest numbered case of those denied cert. Does that merely mean that the Silveira et. al. had to wait the longest before getting slapping in the face?