Probably because of its influence and creativity. Sgt. Pepper probably was not as good an album in terms of how fun it was to listen to as other Beatles albums like Revolver and Rubber Soul, but Rolling Stone explains that it is due to the creativity involved in making it, taking a whole new approach to music on the part of the Beatles.
right, i respect that. but it's still mostly opinion and taste. i could say that koyomon takiki's solo "Drumming for Rain - Live from Titikaka" was the most influential cd ever. just because.