I have no idea WHY ZEN is not on Binance exchange. I sure wish it was. That's where I do the majority of my trading. That's where I prefer to do all my trading. I believe Cryptopia Exchange is a better platform than Bittrex. I at least like the way it looks better than Bittrex. I haven't traded on Bittrex for a while because it was very buggy the last time I was there about 6 months ago. I simply got tired of the bugs and stopped trading on Bittrex. Maybe it's better now. I have no idea.
Binance had Zencash as a 'Community coin of the month' option - twice - and it didn't get the votes needed from the community for the discounted listing. So it has had the opportunity to be listed, but for whatever reason, didn't pull enough votes from people like us.
It could still get listed through the regular channels, but obviously it would be more costly for zencash and might take longer.