The Art Nouveau movement was started in response to a rapidly-changing -- and in the eyes of artists, ugly -- world. So it makes sense that the San Francisco artists designing rock posters, album covers, and the like in the 1960s would crib Art Nouveau's distinct style. They were responding to their own rapidly-changing, and ugly, world. While these free-spirited designers took heavy inspiration from Art Nouveau, they made their art their own by popping the colors to be vibrant and high-contrast, according to this Vox video.
