Because it's not a transition. It's a mirror. It was put in place before the hg migration plan was accepted, IIRC. Where is this migration plan then if it is not in PEP?
The “hg migration plan” is PEP 385. It means moving from svn.python.org to hg.python.org. It is not possible to make code.python.org/hg a mirror of the future official hg repo (a quick search on the hgbook or the Mercurial wiki will tell you how changeset hashes work). The code.p.o/hg repo was a simple read-only mirror provided as a service to the community. Now that the official repo will be a new, clean Mercurial repo, the current mirror will become redundant and will probably be closed. People using that mirror to follow cpython development and/or propose patches will just redo a clone and rebase their patches.
The two sets of repositories use different conversion tools and rules. They have nothing in common (different changeset IDs, different metadata, different branch/clone layout). That would be nice to hear about in more detail. As I understand there is no place where it is described.
The layout of the future official repo is in PEP 385. The layout of the old mirror can be understood by looking at the Web interface (one repo per svn maintenance branch, etc.)
The transition can be complete in a few minutes. The question is how good it will be. As there are no plan, no roadmap, no status
The transition is not just a command to be run, it includes discussion and writing tools before that, and feedback and support after that. What we call transition is the whole process. People have worked and are still working on the PEP, on the repo, on the policy and on the documentation, and it is unfair not to acknowledge that work and say there is no plan. Regards