Not sure, I was no longer involved when those changes were happening.

What I can say is there was a lot of change in the container landscape at the time. Kubernetes flipped the script, Fleet from CoreOS was deprecated, and my guess is amidst the consolidation happening around Kubernetes and the CNCF, rkt became less relevant and just didn't find a permanent home.

Flannel, Fleet, Rocket... does anyone use any of this stuff still? "Move fast and break things" tends to leave a trail of waste in its wake.