I'm not convinced that CCW is that strong of a factor in determining whether and where mass shootings occur, or that any have been instrumental in stopping an active shooter.
Any well-armed attacker who "has the drop" on a crowd anywhere can inflict many casualties before being finally incapacitated; in fact guerrilla warfare techniques count on just this and can be/have been very effective when used against professional soldiers, or in recent cases against police. In many of these mass shootings it's apparent that the shooter cares little whether he's taken down by his own bullet or the cops' or those of anyone else.
I'd think CCW would be more of a deterrent if everyone who advocated it actually carried everywhere all the time, but my guess would be that only a fraction regularly put their money where their mouth is. In order to prevent a mass shooting or halt one in progress anyone carrying would have to be in a state of constant vigilance and alertness at all times while considering each and every person around them a potential threat and watch their movements accordingly, which would be impractical and exhausting to anyone who wasn't a paid law enforcement officer or security guard. Perhaps attackers choose movie theaters or classrooms (of which there are more in abundance than hunting stores and gun shows) not because of the lack of armed resistance, but because of the large number of people who are lulled and distracted by the nature of activities that occur there.