Japan has been doing it for years while US giants hold each other in a stalemate. At least Square is happily trotting forward while Visa and Mastercard miss another Paypal.
Because smaller merchants are more price sensitive, they were simply assumed not to be interested, so they weren't targeted by their merchant acquirers. Instead, the sales folks went to the large national chains that operate or franchise stores across the US.
It's also interesting to note that much of our mass transit trains in larger cities are equipped with compatible terminals (NYC, DC, SanFran, Chicago, Atlanta, Las Vegas). There's only 3 significant systems integrators in the space (e.g. Cubic Systems), so hopefully Google is in talks with them as well.