Either at all, or because it would impose reduced functionality.
Meaning:
* On 4bpp fb, because those are completely unsupported by the C BB
* On 24bpp fb, because it has limited support for that target format (no alpha blending).
* On 8bpp Linux eInk fb with an inverted palette (that's basically only the K4 ;)).
* On Linux eInk devices w/ no HW inversion support, as that would render NightMode inoperable.
That last point means that it's currently *always* blacklisted on PocketBook, because I don't have a PB device, so I don't know how safe HW inversion would be, meaning none of them are marked as being HW invert capable.
(I was a tiny bit hasty with https://github.com/koreader/koreader-base/pull/912 ;).)