Part of the issue with some of the things are licensing ones, the same reason you'll never see the Alan Moore/Neil Gaiman comic
Marvelman (later renamed Miracleman due to the interference of Marvel comics) released again.
There's also moral grey areas (although not legal ones) with things like the NIN back catalogue. Reznor basically wants to give his music out to all and sundry, but he doesn't have any rights for the songs, and there's cases of musicians having to resort to legal action to get the rights to msuic that their publishers are either no longer publishing or have never published, but nevertheless own.
FLAC (or another lossless codec of choice) is also much more obscure than the "industry standards" MP3 and whatever horrible proprietry formats the large companies want to come out with. I'd imagine a fair amount of music is bought for putting on a mobile magic music box thing such as an iPod, but very few of them support FLAC or even the relatively known, but now relatively defunct, Ogg Vorbis.
On a purely thievery level, I also liked downloading other recordings of Wagner operas. In almost all cases the conductor and composer are dead, and I'm not paying through the nose to have a slightly different interpretation of the bacchanal from Tannhauser. In some cases they're not being published still, so that's another issue.