And i think thats one of the big problems.
If they make chromas now,they might miss the opportunity for a functioning zac skin in the future,which would really suck,because they don't have that many options for Zac anyway.
For example if they made a chroma where zac is brown,then they wouldn't be able to make a brown zac skin for him anymore(like chocolate zac),because the skin would look too similar to the brown chroma.
Zacs unique model makes it hard to find stuff that works on him,but it also allows him some opportunitys that other champs don't have,so I'd say it is worth exploring these opportunities first before they start making chromas.