alt Hacker News Brown said everyone needed a better stereo in the 1990s. Factory radios were lousy. But the aftermarket stereos that people bought to replace the factory-issued ones could easily transfer from one car to the next. For example, if thieves steal the factory stereo from a Volkswagen Jetta, they're going to need a buyer with a Volkswagen Jetta, and all a thief can offer is the stereo the owner had to begin with.
Factory stereos got good (and complicated). Aftermarkets got cheap.
The switch of car sound systems from a discrete unit that used a standard DIN form factor and connections to an integrated part of the dashboard from the mid '90s onward (and now into the complex "infotainment" system) kinda killed it as well. You can't just grad a single box out the dash and run. Does mean that car stereos are either un-upgradable, or require 3rd party kits to replace chunks of the facia nowadays though.