Also mostly because as it happens in most hobby projects, people keep wanting to replicate GNU due to the existing software, thus keeping the UNIX cycle alive.
Smalltalk has issues on exporting your software to be run under a standalone way.
On Interlisp, there's Mezzano, a Common Lisp OS, but it needs some tweaks and optimizations.
Oberon UI wise it's the same as Acme under p9/9front/p9port. On Inferno, 9front and Go superseded it in some ideas.
What issues?
In the meanwhile, Project Oberon from the late Niklaus Wirth (http://www.projectoberon.net/) is a good example of a pedagogical operating system (and programming language) that isn’t Unix. Project Oberon was heavily influenced by the Cedar and Mesa projects at Xerox PARC.