DRM and control over the knowledge within. This is why the Internet Archive fought and lost against publishers to lend ebooks; their goal was to be a library, not just a long term storage archive. The industry treats ebooks as a license, but first sale doctrine preserves the right for libraries to buy and lend books out at no additional cost per rental period. And so, they can only collect and vault knowledge until copyright laws change, while others are not constrained to share liberally (Anna's Archive, Z-lib, etc).

If everything is locked up in ebooks with DRM (Amazon recently nuked old Kindles to close a DRM loophole), culture is locked behind corporate paywalls.