It's more to do with convenience. It was more convenient (and cheaper) to just get Napster / Limewire / Soul Seek / other p2p sharing app and download music than buy it online. In the beginning it was a tech-minded group, but the software got better and easier... and the communities grew.

In short order it was not only tech-minded people who did this en-masse. If a platform is convenient for people to do X, it has a good change of also being popular.