That was my suspicion that it was a cost based decision and one made years ago. It is curious though.
I suppose eye care is the same? Or were "NHS specs" always free back in the day? Maybe prescriptions (in England) too is another example
It is a strange one though. I remember discussions with an American friend once who picked up on the fact NHS dental work isn't free despite essentially everything else being so.
Politically I doubt they could get away with it now.