About the apparent predominance of old source singers. It seems that for 200 years the old songs were about to die out and only collectable from the old people. An exagerration I know, but a recurrent theme.

How about that at any one time it was nostalgic old curmudgeons who couldn't stand the modern stuff that the young people liked and prefered the songs their parents and grandparents sang. How about that the collectors were much less familiar with the popular music of the time of their subjects grandparents and so were more likely to let it slip through their filter.