It appears Rancher does not support Kubernetes beyond v1.12 in its 1.6.x product line. Why is this?

Kubernetes dropped support for etcd v2 in its upstream versions that are v1.13 and later. In these versions, it is only etcd v3 that is supported. All Kubernetes versions currently supported by Rancher 1.6.x only use etcd v2.  

As there is no automated migration process from etcd v2 to etcd v3, v1.12 shall be the last major Kubernetes release supported on Rancher v1.6.x.

If Kubernetes stops back porting CVE fixes to 1.12 releases, it shall be a Rancher responsibility to fork and patch Kubernetes 1.12 repo, and deliver the updated version to Rancher customers who are in need of it and have requested such a patch.

For more details on the manual steps for migrating from etcd v2 to v3, please visit this link: https://coreos.com/etcd/docs/latest/upgrades/upgrade_3_0.html