Pretty sure it's because GitHub allows the user to put content at the root of that URL. And it's an entirely separate domain to avoid cookie stealing or whatever.
People use github.io to host sites that are developed on github.com or around projects hosted on github.com.
Although I feel like unless you need to specifically target web or extensive cross platform support, a better alternative for a higher level framework/library is nvrhi. Especially for C++ developers.
And I feel like if you are making for cross platform or web you are likely using Rust or writing your own game engine/library maybe I view of the ecosystem is skewed.
But it's definitely not very mobile devices ready, you can get it to work though.