The why is both in reference to the audience's reaction and to Dylan's response to it.
The booing was actually for the brevity of Dylan's set and the poor sound quality. Dylan's management figured out that they could get people to buy tickets, just so that they could boo the "electric" portion of shows and encouraged the misconception. I don't know why Dylan didn't return for 35 years.
Source: "This Wheel's on Fire," by Levin Helm.