Bob became the New Left's darling by about 1963, because of the powerful protest songs he had written. He was seen as the crown prince of protest, and people looked to him for leadership, but it was a crown he didn't want to wear, so he began distancing himself from it, and simply refused to get involved in political causes pretty much from that point on. He probably felt that saying "yes" to even one such thing was to open the floodgates.

You should have asked Joan Baez to read that note, Bobert. ;-) She would've done it for you, no problem.