There are sites in the SE network that are about a programming language or a tool commonly used for programming, which would make them on-topic on Stack Overflow. Examples: TeX.SE, Mathematica.SE, Emacs.SE, Vim.SE.
These sites have been proposed because people thought it was a good idea to start a new site with a bit more autonomy, visibility and sense of community than a tag or group of tags on Stack Overflow.
Such a proposal could also have been made for Python, it would surely be viable and perhaps more manageable than the current situation with Python both contributing to and suffering from scale issues on SO.
But a proposal wasn't made - maybe because Python is such a mainstream language that people feel it belongs on SO, maybe because there isn't such a strong sense of community compared to the niche topics mentioned above, or maybe for some completely different reason. Now it is too late - with already 1.9 million questions on SO it would make no sense to start a new site only to suffer from cross-site duplicates and fragmentation of knowledge.