Rumour is, Amazon threatened to stop selling CDs for record companies who wouldn't let them sell DRM-free MP3 files. Certainly they used their massive sales presence to negotiate a deal with the record companies.
I think that was the major tipping point for DRM on music. Apple followed after that.
That's not what happened. Around 2006 all of digital music services were failing because thier DRM was incompatible with the iPod. Once Jobs refused to license FairPlay and gave the record industry the alternative of licensing music DRM free they had no choice but to either allow DRM music or let Apple keep a stranglehold on the market.
My understanding, talking from a few folks who worked on that sort of thing, is that they tried. Of course this is all rumours and chit-chat, so I have no idea how true it is.
But the book publishers would apparently be happy for ebooks to go away, Amazon to stop selling books, and their higher margin deals with Borders to come back. So they called Amazon's bluff.
I think that was the major tipping point for DRM on music. Apple followed after that.