I think that adding DRM to music failed because people were used to CDs without DRM so they would have no desire to switch to a format that was less usable. Music companies could require digital downloads to come with DRM, but what is the point if they are selling CDs without DRM? It probably did not help that the company selling most of the digital downloads was apparently not supportive of DRM ( http://www.apple.com/ca/hotnews/thoughtsonmusic/ ).
For DVDs and ebooks, however, have they ever been sold without some sort of DRM?
Don’t forget Sony: Sony BMG copy protection rootkit scandal - Wikipedia
