My guess is:
• Too complicated for many users (or at least more complicated than 3G)
• Introduces possible security bugs
• Drains more power from the devices
• Raises costs, for hardware and programming and support
Whispernet is dead simple (and I assume Sony's will be as well). Turn wireless on, get your stuff, that's it. No passwords, no pairing, no worries when you wander away from the wifi AP.