The average pub landlord of those surviving pubs does nor really want a bunch of folkies sipping orange juice all night. The days of a bar making money purely from drinks are long gone.
Unless an aficionado he/she can make more serving food. A pub providing a venue is providing a "service" not a profit center.

25% of pubs have closed since 2001. 33% since the 70's. I think it is a pretty safe bet that economics closed all these pubs. Breathyalyser, no smoking, cheaper booze from supermarkets, being able to listen to world class performances in the home the reasons are numerous for diminishing audience. This does not directly correlate with a lack of interest however. Ipsosmori is unlikely to survey such a niche interest therefore any participant figures are unreliable to say the least.
The only folk clubs I ever attended not in pubs were student unions and the Surbiton Assembly rooms, so perhaps I show bias.