My long-held theory is it failed for lack of a blurb. When you visited the site, it was hard to figure out what Plan 9 was exactly. Digging into the documentation produced a link to a gzipped .ps file, and after installing Ghostscript, then Ghostview, then some fonts, then uncompressing the file and viewing it, one could read an academic paper describing in arcane language some problems of interest to file systems theoreticians. This state of affairs continued for years. Not surprising at all that it failed to get much traction.