My personal opinion for what it is worth.
I fly a home built aircraft with mode A/C and collision avoidance that requires a transponder in the other aircraft to work. In my local area I regularly spot aircraft which I know, have a transponder but it is not activating the CA equipment so is not “on”. When asked afterwards pilots tend to say, “I forgot”, but if pushed it is the fear of getting caught infringing, which causes the brain fade. The high publicity incident of the Red Arrows airspace infringement in which of the four aircraft involved, only the one transponding was traced and the pilot prosecuted set a very bad example. Encouraging pilots to use their transponders and fit CA systems would benefit us all.
Rod1