Why. This makes me sad. Plain looks great, but Django's strength is its maturity and amazing, enduring community built on contributions from thousands. Forking it will at best split contributions and mean infrequent merges, and at worst means Plain users lose out on Django improvements and Django users lose out on Plain patches.
It seems like Plain could be just a set of Django packages known to work together, and perhaps a new wrapper script replacing `django-admin`, but instead it appears it is a true fork.
Plain basically looks great. I love Django, and this is a long list of things that I'd need on top of Django anyway. Would I use a framework on top of a framework like this? I'm not sure. I just wish it was built in a way that contributed to the Django community instead of one that divides it.