memycelfandeye

Country and Folk were actually considered nearly the same genre for a good amount of time because both were played in rural lands, usually by poor people (though this wasn't always the case obviously) and they had very similar lyrical themes. Hank Williams called himself a folk singer more than he claimed he sang Country. Western, a genre originally considered different from Country, was films created for, well, Western Films. For the better part of the 20th Century Country and Folk were much more similar than Country and Western were.

What caused the change in philosophy to the three genres?

McCarthy more or less.

As you are probably well aware, there was a huge backlash against Communism and most liberal philosophies in the 1950s. Many folk artists were either directly or indirectly linked to the Communist movement (Seeger was a Communist, Guthrie was very obviously a Communist sympathizer) and as more folk artists were being questioned, many members of the Country community didn't want to be seen as questionable to the government, so the genre generally gravitated even more to the Conservative train of thought and culture, allowing the similarities between Country and Western to become so prevelant that they almost no longer existed.