To be fair Matt, some of the examples you give are listed in Time Out every week as well as elsewhere, including a lot of the events taking place at C House, sure its certainly the case that some clubs are more professional than others in promoting their events but as often as not this comes down to time and resources (bearing in mind that the average folk club organiser has a full time job, family and other responsibilities).
Also just to go back to your example of Martin C, every folk club gig he does in London attracts a full room, usually composed of people who wouldnt attend at any other time during the year so they must be finding out from somewhere?
None of which is to say you arent right and that clubs couldnt be better publicised but I do think (at least in the case of London) its a little generalised.