How many average joe's carry an American Express? They need Visa/Mastercard and some well-known retailers to make this work from the get-go.
An Amex card is no harder to get than a MC/Visa. Costco has done quite well in the relevant middle-class-american-suburb market with an Amex-only policy. This isn't the barrier. The retail penetration, however, is the barrier. If you can't use these things mostly everywhere, this service is going to be no more successful than Google Wallet.
The second issue is fragmentation. Look at wireless charging. Two separate solutions. What works at McDonald's won't work at Starbucks (and vice versa). Although both support wireless charging, they support different models. http://www.androidauthority.com/qi-a4wp-wireless-charging-st...
What use is a wireless payment service going to be if 90% of phones can't use it? When there is a competing standard?