With version 4, Proxmox changed the technology providing Proxmox VE's containerization layer.
Up until this release, Proxmox VE relied on OpenVZ.
However, with the release of 4.1, containers are now made possible through LXC.
As far as I can tell, Proxmox Gmbh. has not accounted for this abrupt change.
Perhaps this infographic provided by Google Trends offers some insight: