As usual, much depends what you mean. I just happened to have Lucy Broadwood's "English County Songs" next to the PC. That was published in 1893, and so everything is arranged for piano. This is typical for folk music published before, say, the middle of the 20th century. If on the other hand you mean modern compositions in folk style (trying to avoid raising a certain issue), its probably just that far more modern homes have a guitar than a piano.