They require either complicated trickery on the web server, or lots of additional software modules on the student's computer as well as a browser. Either way, it is difficult to accomplish. However, future versions of web browsers and servers are likely to make it easier. This is, perhaps, where the web excels. With the right initial design, changing materials on a web site can be simple and quick.
Different audiences have different expectations and capabilities.
The great strength of web sites is that you don't need to know in advance who will access the site. Conversely, with papers, floppy disks and cd-roms, you need to know who and where your audience are in order to send them materials. So if your materials are intended for open access in the truest sense, then the web is the best possible medium. It is important to think clearly about whose life is being made easier by different forms of publishing. There may be times when a web site is simpler for the publisher but less convenient for the user, and vice-versa.