Blackcatter: Likely you're exactly right but (and I also 'but' Dani-C and Watson) of the current and past club organizers - you might as well claim that marriage partners tend to be equally divided among the sexes, and for similar reasons - Which sex tends to handle announcements and singing and which food (and other "housekeeping") functions? Division of labo(u)r, role modeling, genetic predisposition & all that.

Re singing in the past era, I have experience of mid-60's Edinburgh where women certainly held their own. But there was and is still a strong tradition of female ballad-singing in Scotland. This leaked over into the clubs.

As to most of the names cited above, most are just pop, commercial singers with an acoustic label stuck on them. Their popularity depends on the same factors as any other pop singers - emotional impact and sexy presentation. This is not an definition of "folk" just a description of the presentation in an urban, commercial setting as opposed to a more traditional one (say, a country pub or kitchen table.)

Could be that the more commercial and/or demanding the 'act,' the more likely men are to be in successful at it and, therefore, in demand - same for philosophers, sports figures, actors, etc.