The short version of the answer, for the same reason your can't run windows from your laptop windows installation DVD on your windows phone, though both are supposedly "windows devices".
The long version, there is way more into a specific piece of software than just its "name". Source code (be it linux, kodi or otherwise) needs to "compiled" into machine dependant executables (that is built for specific architecture CPU/GPU). With that cleared up there is no such thing as either vanilla Kodi or vanilla Linux. The fact is XBMC foundation supplies Kodi executables for very specific hardware + OS combinations (which you can find here: http://kodi.tv/download/
), Team-Kodi doesn't provide any Kodi linux builds that would work on Pivos hardware (or any other Amlogic ARM device). Funny if you lookup arm amlogic-m3 linux builds on Kodi wiki (http://kodi.wiki/view/Linux#ARM
) you will find two links one of which redirects back to this forums (one of Hydra's unofficial builds) and the other is a thread currently doing nothing more than monitoring TOFULinux DS release thread to download TOFULinux builds as they are released, strip them from all Pivos hardware related checks/locks (thus making them installable on the gazillion manufacturer abandoned boxes out there) then rehost them (needless to say in violation of the terms and condition under-which these builds were released).
As to your question regarding affiliation with XBMC.
- Pivos was an official XBMC sponsor.
- Pivos sponsored the majority of the XBMC-Android port (i.e. key development was done by Pivos developers during their paid work hours).
- Pivos provided the XBMC foundation with the development hardware for the XBMC-Android port (namely the Pivos XIOS DS, thus the label "official XBMC development platform" on the DS packaging).
- Pivos contributed the XBMC-Linux for amlogic devices in its entirety.
With all that said, there is nothing that obligates Team-XBMC (now Team-Kodi) to release and maintain an XBMC (not to mention Kodi) build for Pivos devices (and the tons of similar devices out there).
One last thing to note, XBMC and Kodi are two distinct and non-interchangeable terms, XBMC refers to the Team-XBMC "software" builds up to Gotham 13.2, Kodi refers to Team-Kodi software releases past Gotham 13.2. So the phrase "XBMC/Kodi" should be interpreted as "Kodi software from XBMC foundation" not "XBMC software and/or Kodi software".
Now to sum up:
why is it exactly that these Pivos boxes, one of which (DS) I bought because it had been advertised as "official" XBMC hardware, cannot run any stable build of XBMC/KODI that the user wants on Linux?
Because non of these builds are built to work with DS hardware (or any other amlogic box).
Why were "unofficial builds" like Hydra's needed at all?
Because of the answer above.
And why isn't there such an unofficial build for the newest version of KODI already?
Not sure, they are unofficial, Pivos neither support nor endorse these builds in any way (and such builds will void your warranty if your box break with them installed). Pivos just provides a venue for these releases as long as the release developer(s) adheres to the open source requirements (and as long as discussion of such builds remains confided to their release threads).
And might there be?
Don't know. All Pivos code that would assist with completing such port is already openly released.
And if so, why would I prefer TOFU over KODI?
Its a moot question currently, but when/if it happens it will be left to users to decide on their own (given the voided warranty disclaimer).