The TSA claims they simply take too long. Folks like me request the overly vigorous (but slightly less carcinogenic) groping session over the x-ray machines anyway, so I guess you have us to thank for this.
Those backscatter x-ray scanners are still here in Texas last time I checked, although my last few flights didn't require having to go through any or to opt out thanks to priority security lanes that apparently don't require walking through such scanners. So far as I can tell it's mainly larger blue city airports where the x-ray machines are being removed, only to be dumped in smaller and more welcoming red state airports.
Recently I had the option to go through the cell phone radiation style millimeter wave version and didn't bother with requesting the grouping on that one. Less of an issue with exposure and privacy with those models, based on my understanding anyway. In any case based on what I've heard both types of scanners apparently have flaky track records with false positives that leave staff desensitized to real threats over time while also having multiple blind spots that are likely to miss an adversary who is willing to insert a weapon or explosive device into one or more body cavities. Newer versions may be able to fix some of the inherent problems but you'll probably never detect a truly determined adversary who is willing to implant something into his or her flesh in a way that mimics conventional biological abnormalities or deformities.
