Why Isn't There Just One Way to Pay For Things With My Phone?

A bunch of big retailers including Wal-Mart, Target, 7-Eleven, and Sunoco are supposedly getting together to launch a joint mobile payment system called Merchant Customer Exchange (MCX), the WSJ is reporting. But seriously, why another network? Google Wallet is promising, but it's not on many devices, nor is it available at many places. Why can't we just settle on some kind of mobile payment standard? [WSJ]