Actually it isn't JUST poor sales, it is also that despite all the talk about our national pride, the British don't really seem to have any commitment to "Buying British". In my experience, the French, Italians, Japanese and Americans all believe that they have a responsibility to support their own industries.
I seem to recall that there was a time when Great Britain used to have a car industry? Nowadays, we tend to buy most of our cars from countries with which we were at war some sixty years ago - the same is true of motorcycles, where are the Triumphs, BSAs, Nortons, Velocettes, Vincents, Royal Enfields and Ariels of yesteryear?