I'm not very familiar with the details, but I can comment on the results that I see: there's a multitude of python implementations (Jython, ironpython, pypy, stackless python), but only CPython has widespread adoption and there's little overlap between the ecosystems: rarely will python code be written to run on multiple implementations. I think the largest reason for this is that the CPython ecosystem is basically built on a foundation native C code (not just in the standard library but in many of the commonly used packages), and that interface is only starting to get some standardisation, as well as being very tied to CPython implementation details which made it very difficult to implement on other platforms.