They announced dropping the old glibc version half a year ago, so I doubt that’s the reason. Besides, most LTS releases backport security fixes like these to old versions, so there’s no reason to assume an old version number is vulnerable necessarily.
Maybe because of the privilege escalation that was just found in glibc?
https://www.bleepingcomputer.com/news/security/new-linux-glibc-flaw-lets-attackers-get-root-on-major-distros/
They announced dropping the old glibc version half a year ago, so I doubt that’s the reason. Besides, most LTS releases backport security fixes like these to old versions, so there’s no reason to assume an old version number is vulnerable necessarily.