From wiki... "The Beach Boys, who had been involved in the conception of the event and were at one point scheduled to headline and close the show, failed to perform. This resulted from a number of issues plaguing the group. Carl Wilson was in a feud with officials for his refusal to be drafted into military service during the Vietnam War. The group's new, radical album Smile had recently been aborted, with band leader Brian Wilson in a depressed state and unwilling to perform (he hadn't performed live with the group since late 1964, although he would do so in Honolulu, Hawaii in August 1967). Since Smile had not been released, the group felt their older material would not go over well. The cancellation permanently damaged their reputation and popularity in the US, which would contribute to their replacement album Smiley Smile charting lower than any other of their previous album releases."