Think of it like this.
I'm a techie dude, and before today- I'd NEVER heard of Venmo. I'll probably forget the name tomorrow.
So..... take a non techie, like my gran- doesn't do a lot of purchasing online; still hesitant to send all her credit card info into the "cloud". However, she DOES trust the largest tech company in the history of man that secures her data/money with her fingerprint!
If you wanted to try to get her and five of her friends to all download the app you're touting, sign up for it, register their cards, etc, etc. you'd be in for a MONUMENTAL undertaking.
If they could whip out their iPhones & with a thumb tap split the after church lunch bill, you may have a winner!
If the best cross platform app won, we'd all be messaging on ICQ still.
Ease is trump.
Venmo is already used by pretty much everyone I know that would be called young professionals. Think of it like Facebook right before it expanded to non-college users. It's huge and this time next year will have expanded exponentially. Apple might be able to capture your gran, but Apple missed the boat here and is already out on the most valuable demographics.