Operation -- Annihilate! - S1-E30
Dr. McCoy: [In an aside to Kirk] Please don't tell Spock I said he was the best first officer in the fleet.
Spock: Why, thank you, Doctor McCoy.
Capt. Kirk: You were so concerned about his Vulcan eyes, Doctor, you forgot about his Vulcan ears.
Answer: After causing Norman to overload, all of the other androids shut down. The same could be said for the androids on the Enterprise.