One of the reasons would be that it isn't used that often and won't allow access to your network with commodity hardware.
As these devices become more popular, we'll start seeing more Zigbees and Z-wave devices become commodity hardware.
Zigbee has built-in, easy, encryption. You simply set your router to allow unsecured joins temporarily whenever you get a new device, which then learns the key.
When the network is set to allow only secure joins only devices pre-configured with the key can see anything other than encrypted frames.
Of course, wifi has gotten more secure over time to the point that I'd be more concerned about giving the devices internet access than I would about intruders on the network.
In just one highrise project I did in Las Vegas last year we installed something like 65,000 Zigbee devices, and that's not atypical for highrise hotel construction. Zigbee is everywhere in building automation.