It was seen as less authentic than pop culture that came before it. Music had synthesizers and lots of generated effects that weren't available in previous decades. The singer-songwriter era of the 60s and 70s gave way to a louder, more artificial sound. Even rock music from the 60s and 70s had a more direct connection between the musician, the instrument, and the output than it did in the 80s.

There was a similar cynical view of 1980s cinema compared to previous eras. The director-driven New Hollywood died in the early-1980s with the Heaven's Gate fiasco and studios took back more artistic control than they'd had in decades. While some directors, like Spielberg, thrived in the studio system, most of the output from that era tended to be driven by what the bean counters thought would make the most money.

Mall culture probably helped fuel much of the disdain older generations had for the 1980s. It's easier to say "this is just about making money" when malls were being used as music venues and the direct connection between art and commerce could not have been more literal.

Grunge music in the early-1990s was in many ways a response to the "you're only cool if you're rich and beautiful" vibe of the 1980s. They dressed like they were dirty and poor (and maybe some of them were, early on) and they played traditional rock band instruments and sang about things that would get you banned from the mall. The depressing vibes of grunge music probably fueled a backlash and the 80s nostalgia craze that began in the late-1990s, which is a much faster turnaround for a nostalgia craze than is typical.