Because he embodied the voice of prepubescent and adolescent baby boomers, the biggest population balloon in history, sounded (early on) like a spoiled, truculent idealist (which found a lot of sympathetic and kindred spirits in that population) and lashed out against the post-war dizzy hypocrisy of the Fifties, which that population was just breaking away from in their normal curve of maturation. In short, demographics - the right voice at the right moment.

Plus, he had a certain quirky genius which was appealing.

A