At a minimum, it was set by the Japanese Ministry of Education to be used on all computers in schools from primary through college --- if implemented there would have been an entire generation of Japanese students who grew up with that OS.
There was an entire generation that grew up in Japan that used Japan-only cell phones that were used nowhere else. The same thing would’ve happened with this OS, it would be used in Japan but nowhere else.
Yes, but aside from novels (in Japan), not much is written/created on cell phones --- an OS presumably would've had a development environment which might have caused the OS to continue on after education was done.