Hello dear mr/mrs. Now, the ways to achieve new cosmetics are battle pass and phantom market. I am not mentioning the free cosmetics as they are not to be mentioned. As you know, the phantom market price is a lot bigger than battle pass one. 11 000 orbs require a lot of investment. So when you put on a new char class - bk last year or tempest this year, i propose you to make the purchased phantom market able to apply on those new classes. For example - i like some phantom market cosmetics that i bought for my monk but when BK came out i couldnt use them. It was hard because i made big investment in monk and had to throw it as monk is not very popular now. Then i developed BK and bought him phantom market aswell. That was another big investment. Then you made tempest and i switched to it and loved it. But yet again i cannot use both my monk ans bk phantom market cosmetic :(. I believe that if you make all phantom market able to apply to all classes it will make players enjoy and be more satisfy when switch to another class and use their favorite cosmetic on their favorite new class 
My guess is that the designer team is fluctuating, the designers who made the previous skins are not there anymore and the current ones can’t recreate a fitting update for newer classes. The mad king pm skin is probably available for the tempest because that class might have been in the works way before bk (the class has similarly themed legendary essence appearance design than the starting classes and the mad king pm skin was added to the chinese version not long after the game was released, also the backstory could fit well into the stormpoint update patch).
Tbh I could very well understand if designers leave quickly as they are probably the least respected employees there since they’ve already created 604 appearance sets that essentially no one uses. Their work is simply useless currently.
I dont think that it is impossible to be set. After all its a lot of money investment and the game should be a bit more friendly for such investment as it concerns only cosmetic…