Linux was never ported to 16- or 8-bit hardware but through the magic of Turing completeness and emulation somebody did actually manage to get an 8-bit micro-controller: http://dmitry.gr/index.php?r=05.Projects&proj=07.%20Linux%20...

(Yes, this is so convoluted, and so unusable, that it basically reinforces, rather than refutes, your point.)

Well, the ELKS[1][2] project has been working on a 16bit port on and off since the late 90s. At one point Alan Cox[3] was an active contributor.

[1] http://elks.sourceforge.net/

[2] https://github.com/jbruchon/elks

[3] https://en.wikipedia.org/wiki/Alan_Cox