Why is git popular?
Projects outside the kernel began adopting Git shortly after its release. A key landmark in Git adoption came when Keith Packard, one of the lead developers of the X Window System, published two influential articles in 2007.
He saw Git as more robust from an administration point of view, which matters for open source projects that tend not to have a lot of infrastructure support.
After X moved to Git based on Keith's research, a lot of other projects outside the kernel started considering it more seriously as well.