The benefit of wifi spectrum is that the parts are cheap and you generally won't get in trouble for transmitting signals without a license, which is what their system does. (It's not passive.) Unfortunately the wifi spectrum isn't that great for through-wall imaging in general -- you're going to lose a lot of transmission power if the wall is composed of just about anything besides drywall, glass, or wood, on top of the massive loss of power you get by default since the received signal's power is inversely proportional to the distance raised to the fourth power.
Is there a range of frequencies that does better? Don't frequencies that go through walls easily also go through bodies easily, or are there frequencies where the body is particularly reflective w.r.t. concrete walls?
X-rays would do better, but you know, dangerous. On a less sarcastic note, yes, there are frequencies which would be better suited for a specific target material, but like Jach said, regulations and cost are what keeps this things in the WiFi spectrum. I think it also has a kind of "wow factor" when told to non-technical people.
This is a different technology, those are not radars as far as I know, and I'm not sure it's possible to build x-ray radars due to low reflectance/scattering.
My guess is that you need to have the kind of wavelength that meaningfully interacts with human features and body composition (i.e. something that scatters and reflects off of you, instead of being absorbed). It's not specific to the WiFi spectrum though, there are plenty of frequency ranges that meet these requirements.