I've never recommended Zelle. Tales of their complete lack of customer service, consumer protection, and unwillingness to be held accountable are not new. ACH is archaic (and has its own dangers e.g. pull transactions) but it generally works.
I believe that one of the design considerations between the banks that developed Zelle was specifically to move more liability to the customer for fraudulent transactions.
The real reason it doesn't compete is its slow, and many banks have terms of service that say something like "You can only set it up between accounts you own" so sending to other people is often technically a violation of your agreement with a bank.