Because it's rendered directly from README.md.
That way it's easier for me, because I don't have to render it every time I make a change (I make a lot of little edits all the time), and project's Github page (https://github.com/gto76/python-cheatsheet) always has the same content as webpage.
It's quite annoying to have to enable JS for 3 different domains just to see something other than a blank page. It also prevents non-JS browsers and retrievers from seeing content.
If you were to use e.g. org-mode for the source document, you could easily export to HTML automatically when the file is saved.
You could also easily use a git hook to run e.g. pandoc to convert md to HTML automatically.
There are many ways of automatically exporting HTML when you save the md source file. Please use one.