Probably for the same reason that almost all rappers are black or almost all kickboxers are oriental or almost all ku klux klan members are in the USA. It's a result of the gradual workings of cultural influences, spread from person to person, and promulgated through the media of print, radio, recorded music and most of all TV. Like attracts like. North American folk music has had a largely white audience right from the beginning, although it has definitely been influenced to some extent by black music, Native American traditions, and indeed all of the world's cultural traditions. No need for concern. The universe is unfolding as it should.