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…
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.