I hadn't really noticed, but maybe, perhaps, it's to make Arthur's family stand out more? I'm not sure, but it seems like to me if they were to try to introduce any newer characters that were aardvarks, kids might automatically assume that they're relatives of Arthur or something, I really don't know... but it's my opinion that it's probably so the Read family will stand out more. Some shows have a tendency to do that, kind of like M*A*S*H, in the very early episodes, Radar O'Reilly wasn't the only one who wore one of those wool caps, but shortly thereafter, he was, even in cold-snap episodes.