This one is actually more of an ability oddity, but...
Protean, an ability themed on changing how you look at will, is given to two lines - Kecleon's, through the usual cartoonish exaggeration on how chameleons can change colours, and Froakie's, which is based on ninjas, masters of disguise of old times.
So then... why doesn't the Zorua line get it? Sure, it did not exist in Gen V, but what about it being a Hidden Ability? The Pokedex also states they change appearances to trick opponents, which is what the Froakie line also does by changing types as they attack (As per the ninja theme), with the extra power of a STAB boost being just a bonus (Although the whole "changing types to trick opponents" could also be exploited, like Spiking to block Volt Switch or Shadow Sneaking to spinblock).