Strangely enough the matter of "bellows articulation" has been quite prominent in accordion tuition since 1950s

If that's the case, why is it that the overwhelming majority of PA players I know or have seen simply move the bellows slowly and smoothly backwards and forwards and rely on their fingers for articulation?

I only know one PA player who makes effective use of bellows articulation and she plays a small (12 bass) instrument. I suspect the reason is that the inertia in a large PA makes effective bellows articulation impractical.