From TheTruthAboutGuns.com:

The specific reason why the law was overturned isn’t because they were a violation of the Second Amendment (because what self respecting California judge would ever rule in favor of the 2A?). Instead the California Superior Court decision said the requirements were unconstitutionally vague. Thanks to the various pistol caliber carbines available as well as the pistol version of the AR-15 and the AK-47, it makes it impossible to actually define what a handgun caliber is.