I lived through the sixties and can tell you exactly why so many great song writers appeared, social revolution, two very distinct mentalities battled each other during that great decade, the conservatives and the liberals, on one side you had big brother and strict moral values, on the other hand you had a large population of WWII children trying desperately to express there freedom in new and exciting ways.
This was fueled by the Vietnam war, Rock&Roll and the general decline of censorship in the mass media, many terrible as wonderful events occurred that fueled the fire for revolution.
As I see it, singers and song writers came out of the woodwork to voice the concerns about repression by not only the elders of America's society, but also against the U.S. government and there policies regarding freedom of speech and liberty for all.
The sixties were a very turbulent time, African American rights, censorship on TV, the cold war and the battle for nuclear domination all contributed to the distinct separation of both parties.
This caused the Conservatives to staunchly back there own views while the liberals went to the extreme to expand there rights as American citizens.
Unfortunately, there was no effective way to bring this message of freedom to the masses because the media was controlled at the time by the conservatives, therefor the singers and song writers became the vehicle to which to deliver this message of self expression and freedom of the American youth.
There was no middle ground to be taken, you either were for strict moral values or you were not, most of the young people chose to speak out causing a very distinct shift in music and how it was viewed, this in turn brought out the many good song writers out of the woodwork so to speak.
Freedom baby, freedom !