A few simple rules apply to free stuff, especially on the internet.
1. People value something that's free much less than something they pay for.
2. Downloads cost very little on a piece by piece basis. But huge volumes can still add up to a large bill. Free downloads are an opening asking for a Denial of Service type attack or being overwhelmed like the 4th edition site when it went live. See also
http://en.wikipedia.org/wiki/Slashdotted.
3. Preparing the printed product is most, but not all, of the work needed to make a quality electronic version. Printers don't use the same files which are given to the public (they have much larger file sizes for one thing) and the hardcover designs aren't integrated into the same layout.
4. There is added, technical, customer support needed for downloads that is handled by retailers for printed materials. Not to mention added complaints if the thing is down for an upgrade, etc.
5. This won't be popular, but businesses want to establish a relationship with their customers. Confirming that a real person is behind a transaction (by collecting a token amount of money over the internet) is a great boon to one-on-one marketing. I doubt this is more than a minor concern but it can help a company help their customers better.
I'm sure I've forgotten something so others can add to this list.