If memory serves.. Idol was meant to be more Eagle Wardens support, fueling their 'having influence bonuses during high noon only' and not a static bonus, so it didn't make sense to give it the attire keyword. Also, visually it didn't look like an attire. About those visuals..
With ballot counter, this was the first of its kind, and before the keyword had existed. Pb1 brought in Outlaw mask, Sb4-6 brought in Fancy New Hat and Dog's Duster, which were intended to be pieces of clothing to boost your influence. I think that's the loose history of how it happened.