If it was just a 1/1 in front of it (I'm assuming TWI was in the aquatic lane), then it's because it didn't do damage. Deadly will "destroy any plant or zombie that this hurts." Hurts being the key word. The armor of tough beets kept it from actually taking the damage required to activate deadly.
Another good tip in my opinion if they play deadly cards, use team up cards or briar rose to protect your plants, or if you are grass knuckles or citron, use their shield ability.