Zac is a tank/fighter that relies on items to be tanky and both defensive scaling and itemization aren't that strong right now. He's not like an Amumu, Malph, Naut, or Leona that can initiate mid game on little to no gold income.
He doesn't offer more damage, CC, or tankiness than other tanks, particularly early-mid game. His initiation is god-tier though. For Zac to come back, Riot will have to either buff his early game sustain or make pure armor/mr good again. I'm betting on the former since defensive itemization hasn't really been strong since season 2 ended.
None of the health cost champions are doing particularly well right now to be honest. Vlad is probably doing the best out of all of them and that's because he can build mostly damage and get away with it.