Because (sadly) compatibility with the existing software ecosystem and support for common hardware is king, at least if we're talking about a general purpose OS. Who cares about your project if it can't run the N million line behemoth the modern web browser is? With all its deps...

But I'd really love to have an OS that sweeps the pile of legacy cruft we've built up and replaces it with something simpler & smaller. In a sense Plan 9 is just that. But I'm sure one can go further down that road.

Some of Google's internal architecture is inspired by plan9, e.g., gfs. Plan9 also included a language called Aleph which has today evolved into golang.

Union mounts have been particularly interesting: something plan9 implemented, but no one else has successfully.