Why did Zeex choose to use the Ethereum network rather than another network like QTUM?
Ethereum is the most used and trusted blockchain out today. If that changes we will let you know

Yeah, most of the new projects choose ETH blockchain, so it's not questionable which one is the best

I hope that ETH blockchain is constantly improving, because they will certainly have competition and the users will demand higher quality, as the should!
Ethereum team is working on sharding,and that thing should sort all those possible issues,and when on that you add casper,i would say that future is bright for Ethereum,so smart move by Zeex team.