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.
Almost all of my monthly spending is on my American Express.