Rob Landley argues that a lot of the popularity was due to the fact that the FSF had a high bandwidth FTP site at a time when that was pretty rare, so people were willing to sign code to the FSF and license it GPL in order to have access to that distribution method: https://landley.net/notes-2010.html#19-07-2010
Obviously that particular advantage is no longer relevant.
"The GNU Project" is not really a thing. It's just a list, and the list includes projects with varying degrees of connection to the FSF:
1. Some projects are owned and controlled by the FSF. All contributors have to assign copyright to the FSF, and the FSF makes decisions about appointing maintainers, where the code is hosted, etc. Examples: Emacs, gcc, glibc.
2. A much larger set of projects are not FSF-owned or controlled in any meaningful way. The maintainers (or former maintainers) voluntarily chose to associate with the GNU project, but didn't assign copyright to the FSF, and basically kept their own project governance. Examples: R[1], GNOME[2], GIMP.
3. Finally, many projects on the official list[3] are basically abandonware. They got started because someone thought it was important or useful to have an open source clone of some widely used piece of software, but they never got close to feature-complete, were never widely used, and the authors have moved on. Check the most recent releases for most of the software on the list - it's often been 2+ years since there has been any activity.
The projects that have moved to GitHub are all in category #2. The developers and maintainers of those projects don't necessarily share the antagonism towards SaaS platforms that the FSF has historically had, and they've moved to a platform with better tooling and a larger community. A few better-resourced projects that have stronger views on avoiding proprietary SaaS code have set up self-hosted GitLab instances. The projects I follow that have FSF-appointed maintainers have stayed on GNU Savannah, although many have GitHub read-only mirrors.
Obviously that particular advantage is no longer relevant.