In the end all your computer understand is machine code, it does not care if you used C, Assembly, Java, python or qbasic.
The main problem with making a OS in python, is that it need alot of conversion to turn to machine code and that takes time crucial for function that must be performed in a fraction of second. Speed is the main issue.
I think that projects like Cython, can take python closer to that direction.
http://www.cython.org/
What Cython does is that it permits you to write pure python code but also use C/C++ features like static typing , pointers , manual memory management etc. to boost speed up to C speed . Then code is automatically converted to c code which can be compiled like any c code. Benchmarks on Cython show that it perfromes from 30% slower than C to 30% faster (more often slower though).
And Cython is not the only thing, it comes from Pyrex. And for those that have been using Puppylinux they may have heard of Genie , a python kind of language that is compiled.
http://puppylinux.org/wikka/ValaGenieintro
Genie like Vala are children of the gnome framework
http://live.gnome.org/Genie
So it seems that there surely motivation.
The question remains of course why we need a python OS ... I think that has been already answered with Linux Because it much easier to maintain and write code for. In Linux python started for very simple scrips and now moves to lower and lower levels , slowly but progressively.
And the fact that projects like Cython are supported by mighty companies like Google. Its clear that loads of programmers do want to replace C with python . Its not a easy task , it wont happen tomorrow, but it certainly move toward this direction.
Its no longer a question if it will happen , it is a question of when it will happen.