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Tons of reasons, really. Unix was originally given away, but then it got wildly popular and Bell Labs regretted they couldn't capture much of the value it created. When Plan 9 came around, they were much, much stricter about it, and so it was much less accessible. Unix was possibly the first widely-available operating system with its particular ethos - frustratingly simple and hackable. Plan 9 has much the same ethos, but wound up having to compete with Unix, which had a decades-long head-start. Possibly the biggest problem Plan 9 has, though, is how well-designed it is. Unix was probably well-designed in its early days, but by the early 90s when Plan 9 came around, it'd been hacked up and contorted to all sorts of use-cases the designers never intended, like networking and graphical terminals. Plan 9 re-unified all these use-cases into a small set of elegant, orthogonal features... but it made anything outside those features even more awkward and difficult. For example, a web-browser doesn't make much sense in a Plan 9 world - you'd just run remote applications and have them display locally, rather than having a special local application that all remote applications must run inside. But it's hard to do anything in the modern world without a web-browser, so Plan 9 isn't a practical alternative for most people. |
By not making the operating system abstractions similar to other existing operating systems all popular applications had to be rewritten for Plan 9. There was a network effect of existing apps that worked well enough and developers were already familiar with the existing systems which made it hard for users to switch over.Plan 9 was a better operating system technically but it didn't have the utility that existing operating systems already provided to users.
Non-computer people would also probably just use whatever OS the computer they bought had. Using something outside of MacOS or Windows was not normal for a non-technical person in the mid 90s.