Why isn't device XYZ supported

Why isn't device XYZ supported?

Typically because https://plex.tv doesn't support it. Plexupdate is only a tool to automate the download and installation of files from the https://www.plex.tv/downloads/ page. If your device isn't listed as supported on this page, then plexupdate.sh cannot help you.

But I really really want to run on unsupported devices?

I recommend you checkout https://github.com/uglymagoo/plexmediaserver-installer which apparently can produce deb packages for Raspberry Pi devices (and possibly others) but this is NOT OFFICIALLY SUPPORTED BY NEITHER PLEX.TV or PLEXUPDATE and no support will be giving other than referring you to this wiki page ;-)