The important question here is, why? Was it because of those features? Or something else?

I don't know, but one major factor in OS adoption is network effects. "The rich get richer", and it's very hard to introduce a new and different system, no matter how good it is.

Unlike Unix, which was basically free software and therefore extendable and available for companies to build their own versions of, Plan 9 was proprietary from Bell Labs.

Ah, Plan 9. Vastly superior to its predecessor in every way, except the one that counts: bums on seats.

And, for a long time, price/licensing.