Why is Zac such a low picked champ?

He has no dmg and is super reliant on allies

Because you have to rely on teammates to follow up on the E. Requires teamwork that's outside of your control, not ideal for lower MMR. Playing him is very frustrating when you nailed a good engage but your teammates did not react.... despite the long E charing up window and/or pings. To many folks just hang back until after you landed before making a play.

And if you don't have good zac mechanics your just feed, not everybody's down to invest the time for that.