It's a commercial product owned by Microsoft designed to profit - financially - directly off a piece of software Linus Torvalds himself wrote specifically to manage the development of Linux. Not supporting Linux - when they trivially could - is an outright statement.
The GitHub site itself is commercial, but unless I am missing something the GitHub Desktop client in question is actually open source (MIT) https://github.com/desktop/desktop.
In fact, as was mentioned elsewhere, there is an actively maintained fork of the project that adds support for Linux environments. https://github.com/shiftkey/desktop IMHO this is the open source ecosystem working as intended!
