Canonical has taken stewardship of LXD, the extension for managing LXC (Linux Containers). The Linux Containers team explicitly pointed out that everything else LXC will remain under its stewardship: “Canonical, the creator and main contributor of the LXD project, has decided that after over eight years as part of the Linux Containers community, the project would now be better served directly under Canonical’s own set of projects.”
As to the reason why, it does seem to boil down to the fact that over the last several years, there has been limited development of LXD, and that under Canonical’s guidance, it can more tightly integrate it into the various versions of Ubuntu, and extend the functionality to enhance the whole container experience on Linux.