Actually they didn't - I think that in every election after 1945 more people have not voted for the winners than have.
Anyway - STI, the answer to your question is simple, and it will clear up all of the inconsistencies you see.
There is not a lot of money to be made by taking control of the installation of lights on the outsides of houses.
There is a lot of money to be made by taking control of electrical installation work in kitchens and bathrooms, either by preventing kitchen fitters, bathroom fitters, plumbers etc from doing it, or by making kitchen fitters, bathroom fitters, plumbers etc pay you to be allowed to do it, and that was the motive behind NICEIC and the ECA lobbying for statutory controls.