A) The off-duty officer wasn't some random ccw person, she was actually working security at the complex.
B) Uniformed Police were already on scene.
C) The gunmen was essentially committing a crime of passion. He went after specific people with seemingly little planning, a hand gun, and likely far less ammo and armor than recent mass shootings.
The headline you want from the media?
"Jilted Lover Shoots Girlfriend And One Other Before Being Shot By Police"
Now, in your world where there should be multiple ccw's in movie theaters, what happens when the police enter the theater and there's 5-6 random people exchanging fire???