Because you were guaranteed that what you were downloading was what it said it was, and was high quality, and would have the correct tagging and album art and all of that.
It’s been shown repeatedly that a large part of piracy isn’t about cost, it’s about convenience. It was easier to pay $0.99 and get what you wanted when you wanted it, than download 8 files off of Napster and hope that one of them was actually a decent bitrate and was the song the title said it was.
Back when eMule was a thing, it was super common to spend an entire day downloading a 700MB video file at 5kb/s, only for it to be Fight Club instead of whatever you thought you were downloading. It’s the same thing with music.