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.)