Your stated reasons aren't valid. I think they ported Mozilla until 2002 (Russ Cox, maybe?). There was also a compat layer: APE.
As a counter to your argument: a system that has little capability compared to the well-established alternatives, starts from nothing, yet takes over the world: Linux.
I really believe there was no greater reason for Plan 9's failure to catch on (or at least it's ideas) than my peers not being interested in their own education.