Perhaps because it foresaw that the UHA would be over turned if access to handguns was too tightly restricted?
Just my opinion, but I seriously doubt that the didn't require microstamping for revolvers because they were concerned it would be going too far.
I think rather it was an uncommon exercise of good sense. Not that they wanted to, but rather because they didn't really have a choice, if they were to have anyone believe their claimed justification.
Which was, that microstamping marks the cases (actually the primer) with the ID of the gun that fired it, and was needed in order to aid law enforcement solve crimes.
But not even those slickmeisters could come up with a believable excuse why revolvers (which do not leave cases as evidence at the crime scene) needed microstamping, because if the police don't find cases at the crime scene there is NO benefit to the investigation. Requiring revolvers to have microstamping strengthens the argument that the law was about gun control, and not "just" about helping the police, which was the argument they wanted the public to swallow.