This [0] may provide a hint. Heimdal was developed outside of the US and not subject to export restrictions, unlike MIT. So perhaps in the beginning it wasn’t the package of choice to begin with.

And this [1] says for interoperability reasons.

[0] https://docs-archive.freebsd.org/doc/11.1-RELEASE/usr/local/...

[1] https://freebsdfoundation.org/project/import-mit-kerberos-in...