To be practical, Mosaic or Netscape by itself does not provide an appropriate means to view every kind of graphic or sound format for every different kind of hardware. Instead, when the browser downloads a particular type of file, it looks up that type in a table, and runs an independent program to view or play that file. You can add hardware or change the available viewers, and after you update the table, you can still use your original browser program. Thus, the browser can continue to act as the core client and deal with file formats and viewers that were not even envisioned when it was written. The fact that good WWW browsers are now becoming widely available has probably turned the tide in the long run.
Good hypertext is hard and very labor intensive.
A human brain has to decide how to mark the text, and that takes time. Sure, there are automatic conversion programs for many formats, but a good, clear document design still depends on real Homo Sapiens, not just software. It is much easier to make plain text available, and gopher has a natural simplicity in dealing with plain text. The Gopher menu format might seem rigid, but it forces every entry to have an associated title. Reasonable titles are clear and descriptive, and they can be searched by programs like Veronica.