The main reason is... context switching, also i'm a big fan of python's standard library, but I have yet to do any complete work just with it.
But Translating something like Django is out of scope for all the transpilers, it would mean also converting sql drivers and so on? Does this really make any sense? Maybe you have a look at emscripten https://github.com/kripken/emscripten/wiki
Does multiple inheritance
It does multiple inheritance with a polyfill and overally is much more "fatter" than JavaScripthon.
But with JavaScripthon you can use any tool available in JS land to accomplish the same goal. Want to use Backbone's extend()? Use it.. want to use react's createClass() mixin?, the same