The piano is clearly a traditional music instrument, given that it's been used for playing traditional music for about a century. My only problem with it is that, with the exception of the blues, I just don't like the sound of it very much for accompanying traditional music. I think a lot of other people feel the same way, and that, along with the difficulty of transportation, cause it to be less popular than the guitar for trad music. Too bad for me, since I'm a better piano player than a guitarist.