There are multiple reasons. But the first would be that many average people unfortunately have little to no concern over thermal vision based weapons systems. They believe that all of these weapons reside in the hands of governments, governments which supposedly have the best interests of the citizenry at heart. Why should they develop or purchase a thermal evasion system when the "good guys" have all the thermal toys? This is blind faith in an establishment which has not earned our trust...

Sadly, with the advent of numerous privacy and liberty crushing programs, it is safe to say that not all governments if any governments have the best interests of the public at heart. In fact, many governments today are training to subdue and control their respective populations rather than protect them.