Because the U.S. population covers so many ethnic groups, there are just as many types of folk music being preserved as there are ethnic communities. It depends on where you live, and if there are groups in your area preserving this music.You may never be exposed to many of these types of music if you aren't in the right place at the right time. As Joe points out, it is not propagated by the mass media, and you have to look harder for it. Then you have folk music that is not part of any particular emigrant ethnic group. So, it's all there: Polish music, Irish, Scottish, Caribbean, bluegrass, old-timey, you name it, not just the type of folk music the mainstream media latches on to when they latch on to it at all.
An interesting phenoma is the attention given to a particular type of folk music after it is shown in a major motion picture, or in another form which reaches a mass audience. Witness the wider interest in roots music after O Brother Where Art Thou was released, and in sea music after Master and Commander's release. There IS a wider appeal to folk music once people are exposed to it, though it will never be as popular as pop.

You might want to see the Alan Lomax documentarty on traditional music up and down the Mississippi. Someone help me here please...what is the name of that? "Deep River Of Song" I believe.

Chanteyranger