It was a rough night for rock. Megadeth, the Grammy winner for Best Metal Performance, walked to the stage to the tune of “Master of Puppets,” a song that’s actually by another band, Metallica. Before Metallica’s own performance, actress Laverne Cox introduced the band only as “Lady Gaga,” who was onstage, but in a supporting role. Finally, Metallica frontman James Hetfield kicked off the song by singing into an unplugged mic. The trinity of tiny humiliations served as an easy metaphor for the long demise of rock, a Viking funeral for the former king of American music.

A quick glance at the Best Rock Performance category shows how withered the genre has become. David Bowie's haunting 9-minute “Blackstar” took the prize with instrumentation that sounded more like anguished jazz than rock ‘n’ roll. Meanwhile, none of the other four nominees were traditional rock recordings. They included “Don’t Hurt Yourself,” off Beyoncé’s genre-spanning Lemonade; “Heathens,” a pop song that is mostly rapped, by Twenty One Pilots; a cover by Disturbed of “The Sound of Silence” that originally appeared on Conan, and a live performance of “Joe” by Alabama Shakes.

Just a generation ago rock dominated the music landscape. By the 1990 Grammys, the genre was so stuffed with popular artists that there were three separate awards for Best Rock Vocal Performance—for Duo or Group, Female Performer, and Male Performer—plus additional awards for Best Rock Instrumental Performance, Best Hard Rock Performance, and Best Metal Performance (in fact, Metallica won the latter for “One”).

How has rock become so depleted? You can start by blaming the year 1991.

Two years ago, a group of British researchers published a study that charted the evolution of music styles and timbres by looking at 17,000 songs between 1960 and 2010. They charted the rise of Motown in the 1960s, the brief reign of drum-machines in the 1980s, and the spate of weepy love ballads in the 1990s. Among their many findings was that the rock genre, so dominant throughout the 1970s and 1980s, took a sudden nosedive in the early 1990s. In fact, they determined that one year, 1991, marked “the single most important event that has shaped the musical structure of the American charts."

What happened in 1991? Between 1958 and 1990, Billboard had constructed its Hot 100, the list of the country’s most popular songs, with an honor system. They surveyed DJs and record store owners, whose testimonies were often influenced by the music labels. If the labels wanted to push AC/DC, they pushed AC/DC. If they changed their mind and wanted to push the next rock release, AC/DC would fall down the charts and the new band would take their place.

But in 1991, Billboard changed its chart methodology to measure point-of-sales record data and directly monitor radio air play. As I wrote in a 2014 article in The Atlantic, this had a direct impact on the sort of music that made its way to the charts and stayed there. The classic rock and hair-band genre withered in the 1990s while hip hop and country soared up the charts. In the next 25 years, hip hop, country, and pop music have carried on a sonic menage à trois, mixing genres promiscuously to produce the music that currently dominates the charts. There is hip-hop-inflected-pop (Justin Bieber), country-pop (Lady Gaga), and country-rap (Florida Georgia Line and Nelly).

The recent British paper on the last half century of music found that hip hop has reigned the Billboard charts longer than any other musical style. Why might that be? In the early 1990s, some cultural critics argued that rock was qualitatively superior, because rap songs were mere “bricolage.” But it’s precisely because hip hop’s nature is to absorb other musical styles that it has proved so durably elastic. Today’s most popular hip hop artists—like Beyoncé, Drake, Chance the Rapper, and Kanye West—sound very little like the styles that replaced rock in the pop music pantheon in the 1990s. They are more polyphonic, with more diverse inspirations and richer instrumentation and production. Meanwhile, 2017’s Metallica sounded a lot like 1990’s Metallica—even after they got the mics to work.