Since Zac's Release
Soraka has had 3 skins.
Ahri has had 2 skins.
Annie has had 2 skins.
Zac has had 1 skin.
Every champion in the game that was released before May 2014 has at least 2 skins except Zac.
Zac is now a powerful, META jungler that will probably soon become top tier. I don't see what's so hard about releasing a skin for him now that it has been 2 years after his release.