The process namespace was the real innovation
I remember when Plan 9 first went public.
A services-based microkernel? That was a departure from the norm, particularly the monolithic fixed-at-build-time kernels of OSes like *BSD; AIX's ability to dynamically load kernel modules was forward-thinking when it was introduced. But microkernels were already popular in the research community, and by '92 Mach was well on the way to being a true microkernel design.
The Plan 9 team made hay of the filesystem-based IPC mechanism, but we'd had essentially the same thing in UNIX for years, with UNIX-domain sockets (BSD) and the lesser-known FIFOs (AT&T V7, I think). It just wasn't so widely used. Promoting it in Plan 9 was a Good Idea, but not revolutionary.
What really made Plan 9 different was the per-process namespace. So rather than all processes seeing the same filesystem, modulo chroot(), each process would see its own version. And since pretty much everything was pushed to the filesystem as the One Abstraction to Rule Them All, that was important.
But of course in the early-to-mid-1990s it was going to be very difficult for an upstart OS to gain traction, even in the embedded market that AT&T initially targeted for commercialization. The big commercial UNIX players were fighting hard both among themselves and against various BSD distributions, against OS/2, against minicomputer platforms like the AS/400, and then against Linux and Windows. Few people wanted to bet on an outside chance. Linux and Windows were able to muscle their way into the mainstream – taking most of the market from the proprietary UNIXes – but Plan 9 really never had a chance.
There was also widespread perception that the performance cost of message-passing microkernels wasn't worth paying. This was the same period when Microsoft got rid of Windows NT's HAL, for example, and there was a general move back toward pushing all kinds of crap into kernel mode. Throw away the seatbelts to save weight.
Plan 9 might still be a nice choice for doing higher-end embedded stuff like routers, though.