It can combine sound, text, and pictures in ways unique to the medium. It can present absolute piles of information without intimidating or discouraging users with its bulk. Any text on a computer screen is more difficult to read, and on some screens, downright dangerous. The subset of material displayed is very small. Remember, frustration reduces learning.
Readers have to learn to use the computer before they can read the document.
There just has not been enough study on the subject to determine which medium is best or if they should be used only to complement each other. In any case, the most important consideration in producing hypertext is a professional writer conversant in Learning Theory and readability, and expert in the special needs of referent text. People who aren't expert in these areas can, and do, produce the most confusing and chaotic hypertext, much like the first users of desktop publishing produced things that looked like a public restroom wall. However, a mess is a mess, in any technology, in any area of life. So the first step in writing hypertext documents is to organize the material for that medium and to know when and where to use it.