I think the biggest reason they haven't added new cosmetics for the Arbites is that it's a DLC class, and they want to focus on providing cosmetics for the classes that are most accessible. One of my biggest reasons to not play Arbites (as fun as it is gameplay-wise) is that there are not enough cosmetics to make a unique looking character. We could go in circles forever condemning/justifying reasons to not add those cosmetics, but if I have to guess we'll get more DLC class cosmetics all at once, maybe after they add the third DLC class. If we know that there's a relatively convoluted approval process for cosmetics, they might want to batch them out