Although more device 'Technically' support Google Wallet, the history of Wallet is a troubled one.
Initially the ability to the NFC chipset for mobile payment, to maintain integrity, required that the NFC chipset use a secure element.
The secure element is a secondary chipset that provides writable memary and, executes code in a protected enviroment setting (akin to running software in a virtual machine). Although Android supports NFC, the Android OS itself cannot nativity access the Secure Element. For Android to the Secure Element, an application that requires a need for the Secure Element must be installed with the proper keys. If mutlible attempts to install code onto the secure element are unsuccessful, access to it is permanently disabled (a security protocal to prevent brute force attempts). This was a common problem with the earlier versions of Google Wallet. Another compounding problem is the manufacturers of the devices hold the keys and when developing devices for unique markets or carriers may be subject to withholding that information.
Speculation is due to the development time required for Google Wallet, only one United States Cellular Carrier, Sprint-Nextel (CDMA/ 4G LTE) supports Google Wallet out of the box. The other three United States Cellular Carriers AT&T (GSM, 4G LTE), T-Mobile (GSM, 4G LTE) and Verizion (CDMA/ 4G LTE) instead opted for a different mobile payment scheme based upon a variation of the secure element. Instead of an integrated Secure Element with the NFC chipset, the Secure Element would be incorporated into the Subscriber Identity Module (SIM) card, with a dedicated wire or wire pair to the NFC chipset. This is known as ISIS Mobile Wallet.
Since the troubled beginnings of Google Wallet, advances have led to storing the public/private keys remotely and not on the Secure Element, possibly negating the need for the Secure Element. Therefore its possible that the inability for AT&T, T- Mobile and Verizon is more of a legacy bureaucratic decision more than anything solely against Google Wallet. This statement is justified by the fact that the non-official ROMs Cyanogenmod ROM versions 9 and 10.1 incorporate modified NFC libraries that allows emulation of a contactless card without need of access to the Secure Element. With this, an open source alternative to Google Wallet (withholding the name to prevent a possible interpretation of advertising) runs without encountering the problems that Google Wallet faces.
As a closing thought, has it ever been considered that an anti-trust suit could be filed in court due to the anti-competitiveness nature of the Cellular Carriers and thus preventing advancement of NFC?
If any of this info is technically incorrect please mention (including you Google)