Something that seems to have been almost touched upon in this discussion but not quite explicitly brought up for consideration (unless I missed something in this read-through):

Could MARKETING be one factor that defines an audience?

Bluegrass is a style that is easily sold to radio stations, record stores, etc as a sub-genre of Country (is it called American Country over across the pond?). And if the radio stations & record stores don't want to split it out on it's own, it can be lumped with other now-popular Country stars. But the ability to pigeonhole an album is central to The Music Business these days.

So you get a bluegrass-style piece on an album that gets pushed Up The Charts by promoters, and consumers become familiar with the style - so they go to concerts.

"Folk", on the other hand, has not seen the same marketing blitz in the past couple of decades that you've gotten with Country. Perhaps because it's harder to pigeonhole, perhaps because Record Companies didn't want to bother with the potentially controversial subjects from singer-songwriters, perhaps because "toe tapping music" pulls in more Consumer Dollars than historical or "intellectual" Folk, perhaps for some other reasons. For whatever reason you don't have Promoters trying to sell out stadiums to see a Folk singer. (yeah, there are exceptions)

So maybe part of the difference (Folk vs. Bluegrass) is the Folk audience seeks out music that is not so heavily marketed. And some of the Bluegrass audience didn't realize they liked Bluegrass until they heard it on the radio.

Also (and this has been sort of touched on earlier as well) Bluegrass as a style is fairly narrowly defined. Heck, the term was coined specifically to tell audiences that "this band plays music like that other band"! So the audience has in advance a really good idea the sort of music they'll hear. If they like one Bluegrass band, they'll probably like 'em all. With Folk, unless you know the particular performer you might not have any idea whether you'll like the show. It takes a different sort of person to plunk down some of their limited entertainment budget to go see a show that will probably make them think, but otherwise may or may not be a style they're in the mood for.

Val