I think this happened a long time ago and there is now a lot more variation in the instruments used. You shouldn't forget that the fiddle had, itself, taken over from other instruments, most notably the pipe & Tabor.

One of the reasons for the growth in use - even the predominance - of the melodeon during the "revivalist" 50s and 60s and on is the popularity of the instrument amongst working people. I did an extrapolation based on a survey of a small Cambridgeshire village not known for its music and concluded that (if it was at all typical) there were probably 1.75 million to 2 million melodeon players, mostly playing for themselves or for very localised entertainment, during the 1930s/40s. This surprising figure is to some extent borne out by the huge number of melodeons from peoples' attics which continue to pour onto EBay.

I think there was a large pool of talent and quite a few instruments out there at that time, and the instruments were easy to learn and cheap.

:-)
Ian