The problem is that UNIX, while it reeks, it's good enough for most people to not care about it.
Plan 9 was supposed to be the better designed UNIX all around and it did not even dent it. It wasn't significantly better that UNIX to displace it.
http://www.catb.org/esr/writings/taoup/html/plan9.html
> We know what Unix's future used to look like. It was designed by the research group at Bell Labs that built Unix and called ‘Plan 9 from Bell Labs’.[154] Plan 9 was an attempt to do Unix over again, better.
> The long view of history may tell a different story, but in 2003 it looks like Plan 9 failed simply because it fell short of being a compelling enough improvement on Unix to displace its ancestor. Compared to Plan 9, Unix creaks and clanks and has obvious rust spots, but it gets the job done well enough to hold its position. There is a lesson here for ambitious system architects: the most dangerous enemy of a better solution is an existing codebase that is just good enough.