Actually BHIM is exactly that, a singular app for UPI.
There are detailed guides online for how to use it, done mainly during the demonetization time itself.
The main issue is non of the print media is interested in it.
If they cover the app and its use in the front 2-3 pages, everyone will come to know and start using it. But they dont do that.
Govt could also have pushed the people for this by buying adspace in all the national dailies and educated on the app usage.
But they are also not interested. My ex-colleague does it on his own, he has spent time to educate multiple shop owners and labourers in his native.