Python is a much larger language than javascript with a lot of edge cases in the rules and is highly introspective. This means that any work building a next gen VM for python is considerably harder and requires more work. PyPy adopted RPython (a restricted subset of CPython) to make up for that. You also need to be compatible with a large amount of standard libraries to be useful to a decent amount of python programmers. The end result is that we are going to see javascript environments pass up the various python environments (ironpython, psyco, etc.) before too long.
Python already has Psyco[1] and some people claim[2] that Python+Psyco is faster than V8.