GNOME is a free software project (as self-described[0]) not merely an "open source" project. Therefore, while GitHub's place in the "open source" ecosystem and its contributions to said ecosystem are well known (and worthy of respect), it does not share in the principles of the free software community. GNOME, as a free software project (and a subproject of GNU at least nominally[1]), is inclined to select an option in line with its principles.

[0] https://www.gnome.org/about/

[1] https://www.gnu.org/software/software.html