Not possible to download/clone single (sub-) directories

I just can download/clone complete repos (e.g., “RaspberryPi” or “Arduino”)
but I urgently need to enable users also to download/clone single arbitrary (sub-) directories (e.g., RaspberryPi/qt/btns_wiringPi_toolbar at master · dsyleixa/RaspberryPi · GitHub).
But I can’t do that: I just can delete either subdir, but impossible to download it!
Why is that not possible?

Git and the GitHub Git server appear to support this meanwhile:

You could also use GitHub API to retrieve the partial content of a repo if you wanted to.