Multiple factors (neither a good excuse, just reality):

* Lack of interest for other team members, which translated to doing what they thought was a sufficiently minimal amount of knowledge transfer

* An (unwise) attitude that "it's already set up and configured, and terraformed, so we can just acquire that knowledge if and when it's needed"

* Kafka guy left a lot faster than anybody really expected, not leaving much time and practically no documentation

* The rest of the team was already overwhelmed with other responsiblities and didn't have much bandwidth available

* Nobody wanted to be the person/people that ended up "owning" it, so there was a reverse incentive