With respect to Multics, unix would run on smaller computers. Moore's Law means we keep getting those, so whatever can get a foothold on them has an advantage. The question of "why unix?" instead of one of the other small OS at the time is less clear. Luck may have played a part.
With respect to Plan 9, it had to compete with unix which was already established.
Note that during this time, another OS also became popular: DOS. Like unix, it ran on smaller hardware (actually, even smaller than unix could run on... giving DOS a foothold advantage denied unix). And clearly, DOS had some luck with respect to all the other small OS available at that time, being put on the IBM PC. There were two other OSes available for the PC (CP/M and UCSD p-System). Why did DOS win there? I don't know, but it was much cheaper, and possibly, MS BASIC compatibility had a role. Before the IBM PC, personal computer manufacturers would tout "MS BASIC" compatibility as a benefit because it gave users access to existing software.
Once you establish a platform with applications, it's hard to beat you. People don't want the platform - they want the applications. It's a network effect.
By targeting low-powered hardware, you can get established ahead of others, so that by the time the hardware has grown powerful enough to support a "real OS", you have already won. It's a disruptive innovation.
Comments
With respect to Multics, unix would run on smaller computers. Moore's Law means we keep getting those, so whatever can get a foothold on them has an advantage. The question of "why unix?" instead of one of the other small OS at the time is less clear. Luck may have played a part.
With respect to Plan 9, it had to compete with unix which was already established.
Note that during this time, another OS also became popular: DOS. Like unix, it ran on smaller hardware (actually, even smaller than unix could run on... giving DOS a foothold advantage denied unix). And clearly, DOS had some luck with respect to all the other small OS available at that time, being put on the IBM PC. There were two other OSes available for the PC (CP/M and UCSD p-System). Why did DOS win there? I don't know, but it was much cheaper, and possibly, MS BASIC compatibility had a role. Before the IBM PC, personal computer manufacturers would tout "MS BASIC" compatibility as a benefit because it gave users access to existing software.
Once you establish a platform with applications, it's hard to beat you. People don't want the platform - they want the applications. It's a network effect.
By targeting low-powered hardware, you can get established ahead of others, so that by the time the hardware has grown powerful enough to support a "real OS", you have already won. It's a disruptive innovation.