Multics didn't "fail"; rather, the Bell Labs guys decided they didn't want to work on it any more and went off to do their own thing. Multics eventually reached a useful state and was in use in various places (by such customers as the US DoD) until around 2000, when the last Multics system was taken offline. However, Multics ran on an extremely small set of expensive Honeywell computers, which is why it did not reach great prominence.
Plan 9 lacks adoption for an entirely different reason. It is very portable, and in the earlier days (~1995) ran on Suns, SGIs, Alphas, some 68k machines, and the PC. As non-PC hardware has become less widely-used, the other ports were not kept updated. However, compilers exist for the MIPS, 68k, ARM, AMD64, Alpha, x86, SPARC, and PowerPC architectures, and it is not particularly difficult to port to a new machine. Unfortunately, Plan 9 was only available via closed license until 2000. Had it been given freely from the earliest days, it very likely could have taken what is now Linux's place as the primary free OS.
The reason UNIX is so widespread is because it was the right solution at the right time. It was freely available to universities and, like Plan 9, pretty easy to port. The interface was simple yet powerful, and the introduction of pipes meant you could write stupid programs, then chain them together for powerful functionality. It really served to end the days of "One OS per Computer" and the kind of monolithic programs one found on such systems as VMS, where every program was an island.
The original Unix was put together in a thoughtful fashion with an eye to both hardware efficiency and powerful features. However, it was unfortunately written before networks and graphical interfaces became widespread--those portions were "hacked on" later, giving us network sockets and X, two systems often considered the worst parts of Unix.
Plan 9 lacks adoption for an entirely different reason. It is very portable, and in the earlier days (~1995) ran on Suns, SGIs, Alphas, some 68k machines, and the PC. As non-PC hardware has become less widely-used, the other ports were not kept updated. However, compilers exist for the MIPS, 68k, ARM, AMD64, Alpha, x86, SPARC, and PowerPC architectures, and it is not particularly difficult to port to a new machine. Unfortunately, Plan 9 was only available via closed license until 2000. Had it been given freely from the earliest days, it very likely could have taken what is now Linux's place as the primary free OS.
The reason UNIX is so widespread is because it was the right solution at the right time. It was freely available to universities and, like Plan 9, pretty easy to port. The interface was simple yet powerful, and the introduction of pipes meant you could write stupid programs, then chain them together for powerful functionality. It really served to end the days of "One OS per Computer" and the kind of monolithic programs one found on such systems as VMS, where every program was an island.
The original Unix was put together in a thoughtful fashion with an eye to both hardware efficiency and powerful features. However, it was unfortunately written before networks and graphical interfaces became widespread--those portions were "hacked on" later, giving us network sockets and X, two systems often considered the worst parts of Unix.