The law was allowed to "sunset" and you know why the NRA, or any other group/person with a brain, did not sue - the makeup of the SCOTUS virtually assured that it would stand forever. Many stated at the time that this was "step one", thus an affirmation of the AWB/MCL laws by the SCOTUS would have meant an immediate "step two". As you can see, they are at it again, even though gun crime continued to drop after this law had faded out of existance. The vast majority of gun crime does not depend on AWs or HCMs, it is gang/drug thug related, most often by repeat offenders and committed mainly with handguns.
A reasonable attempt should be made, as I have said often, to designate "GUN OK" on all state issued, photo IDs after a check of the NICS database - that would allow EASY and efficient point of sale checks for guns and ammunition for both big, little and occasional sellers of these items. Universal point of sale access to the NICS database is not a good, or efficient, idea and without a very expensive and invasive national gun registration program one can never assure that no sales/transfers to criminals will occur. Beteer to have a better 90% solution at lower cost, than our current 60% solution at higher cost.