I imagine the wait was mostly that it wasn't deemed necessary to have ZFS on Ubuntu until now.

Containerisation has drastically increased interest in CoW filesystems. Traditionally their only use-case has been for implementing livecds and similar ro + rw layer environments.

For this AUFS has been the gold standard since basically forever. However AUFS has some flaws and lack of features that make it undesirable to use in production container workloads, hence looking to ZFS to fill this gap.