Apple has an effective monopoly on portable music players. They have lost some grip in the past few years with the popularity of Android phones that also play music. However, the iPod Classic owned over 90% of the market for hard-disk mp3 players and iPods collectively owned over 50% of the total market around the time iTMS dropped DRM. The remainder of the market was fragmented between Sony, Microsoft, and other players. That meant that FairPlay was your only option for selling DRM-encumbered music.
That gave Apple tremendous leverage over the music labels. Nobody was going to switch from an iPod to a Zune because it meant giving up a library of DRM-encumbered music. And nobody could sell iPod-compatible DRM-encumbered music without going through iTMS (which was already the only venue for selling to well over half the market).
So they did what any rational business would do: Start selling DRM-free music through Amazon (which people could play on their iPods) and push Apple to drop DRM from iTMS. It was actually quite funny to see everyone patting Apple on the back for "fighting for the little guy."
DRM-free ebooks aren't going to happen for a while because there are still too many vendors. Barnes & Noble, Amazon, Apple, Sony, and others all operate ebook stores. If one ebook store gets too uppity, publishers can just take their books elsewhere without having to resort to the music industry's tactics.
Tor (one of the largest sci-fi publishers) is now DRM-free: http://www.tor.com/blogs/2012/07/torforge-e-books-are-now-dr...
Enough people bitched and complained, and now when you buy music on iTunes it comes with twice the bitrate and has no DRM. Why? Because it helped their sales. I made my first iTunes purchase the week they went DRM free, as their service was more convenient than digging up a torrent for an obscure album.
I think (and really hope) it's only a matter of time before the same happens with EBooks. If I could buy non-DRM epubs, I would do it all the time just for the convenience. It saves me having to dig it up on the internet, or fiddling around in Calibre to get the thing readable.