I read a Spengler column about a month ago in which he declared tangentially that the American folk instrument is not the guitar, but the piano. It was unexpected enough that it has stuck with me. It seemed ridiculous. Yet I think he is right.
One might make a case for the violin/fiddle, used in many types of music and highly portable, as we automatically expect a folk instrument to be. Yet I don't think it reaches the threshold of commonality necessary. Not every extended family has a violinist or fiddler. Not every church or school has one.
The other nominations which might occur to us, the banjo because it is American or the harmonica because of its extreme portability, provide a clue where we went wrong in thinking it is the guitar in the first place. Both are commonly associated with poor people's music, and we think of those as "the folk." I can see why in international terms one would learn to automatically exclude the music of the wealthy and elite from any consideration of "folk" music, preferring the music of the countryside and the peasants, who greatly outnumbered them. But in America, that was not so.
The guitar was not especially a poor person's instrument in America anyway. It was a Spanish instrument, which made it popular in Mexico and the southwest, where cowboys and ex-slaves in those areas picked it up. After that happened, the guitar backfilled to the east in bluegrass and country music, breaking north south at the Appalachians. (Guitars were not always present in early bluegrass.) Among black musicians, the backfill hit the Mississippi River and went north-south there instead. By 1900, one could find a lot of African-Americans along the New Orleans-Chicago route who played guitar, and it worked its way into Eastern cities over the next few decades. But guitar isn't a big jazz instrument even now, and it certainly wasn't before the 60's.
There was some German and English use of the guitar which had made its way into America, but they were largely in more classical circles. No, the guitar a century ago was cowboys, hillbillies, and streetcorner singers. And these were exactly the people who academic song-collectors trained in eastern universities thought were "the folk," and thus, the source of Real Folk Music. The guitar was rather an accident in that. Song collectors also checked in on sailors and prisoners; unaccompanied singers were not neglected, nor were fiddlers - though the latter were at a disadvantage because instrumental music runs along different streams in folk music. There was no pro-guitar conspiracy among the academics, and they might not have identified it as America's folk instrument themselves. But the groups that they defined as "the folk" - and there was a lot of politics in their framing - just happened to include the few groups that also had guitars.
By the 1950's, and certainly the 60's, the reach of guitar as the everyday peoples' instrument swept back the other way. Not only did rockers and hootenanny make it their central instrument, but its place grew even stronger in country & western music at exactly the time that genre became wealthier and more mainstream. Classical guitarists got a boost, because now that there were so many other people playing it, many of those became interested in others who could play it really well.
The piano's history in America doesn't look at first to be very folksy. First, it's not very portable. As a person who has moved several pianos I can assure you they aren't very portable. Relatedly, they are more complicated and expensive than the more portable instruments, which one would think would put them out of the reach of po' folk. You can't strap a pianner on yer back and ride the rails. We think of Irish harps or Scots bagpipes, or Greeks with that whole bouzouki/lyre/mandolin spectrum, all portable.
But wherever you rode those rails, there were likely lots of pianos wherever you got off. They made up for portability with ubiquity. (I have claimed that the largest number in the known universe is the number of times two girls have sat down at a church piano and played "Heart and Soul." That was true for two generations anyway, though it has faded now.)
Even small churches had a piano, or a pipe or pedal organ. Schools had them, and even in a mill city in the 1950's in a lower-middle-class district, it was one per classroom. Quality poor, but piano present. Bars and movie theaters had them; town halls, mess halls, dance halls, and dining halls. During slavery, not many African-Americans could get their hands on one, but by 1900, it wasn't just blues guitars along the Mississippi. There was an enormous variety of instruments, and pianos strong among them. Many, many families had them. Because it was a sign of wealth and refinement in the 1700s (especially back in the old countries), it became a middle-class aspiration in the 1800's and by the 20th C, even poor kids took lessons. Starting in 1900, there were 200,000 - 300,000 brand new pianos sold every year, and the instruments tend to last. That has fallen off since about 1980, and guitars have risen to well over that. That's a more recent phenomenon. One might make an argument that the guitar has become the preferred instrument of "the folk" since 1975, when everyone started moving to a new city every decade or so and moved away from heavier instruments. Recent.
All styles of music are played on keyboard. But hymns and church music were the staple early on and persisted even when the keyboards popped up in brothels and barrelhouses. Immigrants from everywhere sat down at them.