Most "consumers" of folk music are unlikely to invest time and money on performers they don't already know about. So it's necessary to create some kind of advance exposure before a performance. This might include a feature story in the local newspaper (not just a tiny listing buried among dozens of others), radio, or previous local gigs.

The early Newport and Philadelphia Folk Festivals did a great job of creating audiences for traditional performers -- the crowds came for the popular acts, but the workshops and mainstage concerts were peppered with traditional performers. The good ones attracted attention, and some of the people who had heard them at a festival would come to their concerts four months later.

Radio is problematic. It seems like many "folk music" radio programs are skewed away from traditional performers and styles. Yes, there are exceptions, but like the rest of noncommercial radio, a lot tend to avoid "difficult" music that don't generate contributions from listeners. A guest shot by, say, a ballad singer or a Finnish accordian band, won't attract a lot of people to a concert if the program usually plays singer-songwriters and banjo pickers. On the other hand, I suspect that the folk music shows on WFMT in Chicago and KDHX in St. Louis are effective for drawing audiences to traditional music concerts.

The best way to build an audience for specialty music concerts is probably word of mouth. You have to build a reputation for quality, and offer a reliable product. And make sure that your series is known by the communities that would be its natural audience.