1. The flow of information is different. There is no need to spill over to the back page, because a web page can be any length. And you can use hypertext links to jump around to different sections of the article. Doing that in print was once called “programmed learning.”
2. Web pages lack a context. If you jump around in a book, then you know where you are based on the feel of the book. But you could get to a particular web page via links on a dozen different pages on as many sites – and that doesn’t count the results of various search engines. You have to write the web page with the notion that the user may not have seen what you wrote on the previous page – i.e. they just got here.

These are just the tip of the iceberg, but they illustrate that each media has its pros and cons. An author that writes for multiple types of media has a real challenge on their hands. A good resource on this subject is the following book:

Price, Jonathan. Hot Text: Web Writing That Works. 1st. Albuquerque, NM, USA: The Communication Circle, 2002. ISBN: 0-7357-1151-8.