"...dislike of concertinas..."
That's the wildest theory yet, but there may be a grain of truth in it. Consider M Carthy: as a singer he's as weird and uncompromising in his own way as PB; indeed there was a time when his singing was so gnarled by mannerism that PB had to tick him off about it. If he'd played an anglo concertina instead of a guitar, would he have achieved the eminence (and made the comfortable living) he's enjoyed these past forty years and more? I think the answer might well be no.