Actually I think you can also attribute it to (a) the iPod becoming massively popular and becoming synonmous with "portable music player", (b) Only Apple DRM and non-DRM-able MP3s supported on the iPod.

The iPod won the music player market, so if you tried to sell music with DRM you need to either (i) play by Apple's rules or (ii) have mundane tech users being confused when they can't play the music they bought on the what at become THE standard music player.

In 2006, during discusion of the Broadcast Flag DRM, an 82 year old senator was given an iPod and started to realise that lots of DRM wouldn't work with it. https://www.eff.org/deeplinks/2006/01/history-and-senator-st...