My theory is that the TSA intentionally introduced backscatter and then abolished it in order to increase public acceptance of MMW. MMW is "safer" and "less intrusive" than BKSX, so by abolishing the latter, they can make it look like they have addressed the problems. If they had skipped BKSX and gone directly to MMW, then people would compare the MMW to the WTMD rather than to the BKSX, and there would be more opposition. By introducing the BKSX as the "baseline", they have succeeded in getting people to accept the MMW.

I personally opt out of both of them, and will continue to do so. Some people have given in and started to take the path of least resistance, which is their choice, but I will never do so.