If much of the information I received was constantly updated at a WWW site I could receive the latest when I needed it. I get the information when I need it. Intranets are cheap. The networked PCs are all perfectly capable intranet clients and the browsers are cheap or free. Intranets are cross platform.
Intranets are the easiest way to get everyone communicating.
Intranets are robust. Much of the underlying technology has been in use on the Internet for one or two decades. Intranets are fast. The speed of the intranet is as fast or faster than accessing the Internet. Once companies discover how useful intranets are, they'll start thinking about making some of the content available to customers on the Web.