For clarity's sake, we need to make the distinction between ZigBee (the radio technology) and Smart Energy (the application-specific profile). While 6lowpan, or wifi, or bluetooth, or zwave for that matter all offer competing radio technologies, each with pros and cons, none of them offer a standardized protocol for home area network devices to trade data such as price, consumption, messaging, etc.
That's where ZigBee has the lead now. If I build a 6lowpan device today, I have to invent such a protocol - this means it won't be compatible with any other device unless they happen to use my proprietary protocol too. Any device with a ZigBee Smart Energy implementation however will work on any Smart Energy home network, letting you mix and match devices from different vendors. That's a HUGE difference in terms of readiness for large scale rollouts and consumer adoption.
So far, the Smart Energy standard has also been branched out to HomePlug. It may end up coming to other radio technologies in the future, though that remains to be seen.