According to Steve Jones, a distinguished professor of communications at University of Illinois at Chicago, the absence of digital booklets can be attributed to two things. First, given all of the different platforms on which people consume digital music, juggling the different template standards for each can be cumbersome. Second, people aren’t looking at digital booklets because we aren’t listening to albums like we used to. “You don’t bring it home in a container and listen to it and look at the sleeve, read the liner notes, et cetera,” said Jones. “You buy your music or stream it instantaneously, usually while you’re doing something else. The space in which one would have looked at the visuals has gone by the wayside.”
I think these are good points, but there are a couple of other things we can point to. For one, the any creative effort or marketing money that would have gone into making booklets has most likely moved to crafting a social networking presence. Social networking is probably a more obvious method for connecting with fans and sharing information.
However, I would guess that part of the problem is that early attempts at digital booklets were so poorly executed that it has poisoned the well. I remember a time shortly before MP3s really took off, and they had started putting this kind of information on a data section of the music CD. That is, if you bought a music CD and put it into a computer, it would show up as a data CD with a terrible Flash application that would auto-play. The apps were poorly thought-out, annoying, and often didn't even include the information that would be in the liner notes (e.g. lyrics). It often seemed like the booklets were only put in there so that the disc would be recognized as a data disc, making it harder to rip the disc and convert them to MP3s. (This was when the record industry was trying to prevent MP3s from becoming mainstream, arguing that it was illegal to rip CDs.)
Over the years, I've only seen one attempt at this sort of thing that didn't seem horribly designed and stupid. A few years back, there was an iOS app for Bob Dylan that was meant to accompany a recently released anthology. It came with some free information, and then for each Bob Dylan album it detected on your device, it would unlock information about that album (or something like that). I don't think it's even available anymore, but I found a video of it.
It wasn't perfect. The interface was still a little wonky in places, and it wasn't in some kind of universal format that you could view on any device. However, it was clear that someone had put in the effort to collect a bunch of photos and information, including various interviews and new content created for the app. They'd at least made an attempt to make an interesting design, and have it somehow connect with the music (e.g. you could listen to the music and it would show you the lyrics currently being played). At the very least, it was interesting enough, and had good enough content, that a fan might find it worthwhile enough to spend some time exploring. I haven't seen anything before or since that seemed like the people who made it had any interest in making it good.