Two thoughts: (ignoring the "folk" thing!(
In the UK in the '50s when the skiffle/folk/rock 'n' roll boom started there must have been an influx of cheap guitars, after postwar austerity
Bert Weedon's "Tune a day" guitar tutor is quoted by most musicians of the era (whatever their later genre) as what got them started.
And a third factor may have been the influence of guitar-players in US forces stationed here during and after the war.
I agree with Bert that before the mid-50s the piano, or accordian, sometimes mouth-organ, was the most likely accompaniment to domestic sing-songs.
..though one of my grandfathers had banjos and mandolins between the wars when in funds.
RtS