Most bands who become popular are never able to repeat the quality of the album that put them on the map. A few examples are Pearl Jam, Nirvana, Smashing Pumpkins, Stone Temple Pilots, Weezer, Fiona Apple, Guns n' Roses, the Violent Femmes, Zero 7, Franz Ferdinand, even Britney Spears and Miles Davis. A few of those had earlier albums, but they were pretty much unknown until their big second album. I am willing to bet that a more scientific sample of popular music would show that subsequent albums generally don't do as well as the big first album.
I have two explanations for why this might be the case. First, and most obviously, the second album after becoming a star is more costly to create with fewer benefits if it's good. When someone has become a rock star, his or her time is more costly. The new star can spend it either by making a lot of money by touring or by enjoying the benefits of their stardom by doing whatever rockstars do, which I suppose is purchasing castles and entering celebrity golf tournaments. Musicians also have less to gain by making another great album; they are already stars and people will buy the next album regardless of what goes on it. Since working hard to create the next album is much more costly with fewer potential benefits, it would be a surprise to find musicians working hard to make it as good as the first album.
The second reason for the dip in quality is that artistic output is probably somewhat random over a group's lifetime and a group is more likely to become popular when they get lucky and create something unusually appealing. Even if stardom has no effect on artistic output, it will appear that quality has dipped after the "breakthrough" album. Consider Nirvana, who released three major albums: Bleach, Nevermind (the breakthrough album), and In Utero. Bleach did not make Nrivana stars, and In Utero probably wouldn't have if it had been their first album. No matter what order the albums were released in - B-N-IU; IU-N-B; N-B-IU; N-IU-B; N-B-IU; IU-B-N; or IU-N-B - it would appear that the quality dipped after they became stars.
I have usually attributed the one hit wonder effect to reason number one, but I suspect that most of it is the more mundane, but somewhat interesting, reason number two. My next post will be in August hopefully after I have passed the bar exam.