Here's a design doc about their OAuth choices: https://github.com/kanidm/kanidm/blob/master/designs/oauth.r...
It would seem simpler to go with the Ory approach of "best in breed" for, say network management tooling (most of which they already have implemented), and then integrate with Keycloak, Okta, FusionAuth, the Ory suite, etc for user management. Maybe they didn't want to do that because there are synergies with integrated user management? I dunno, seems like there are a lot of user management tools out there.
I also find it interesting that they explicitly disallow a goal of building a better LDAP server. I think there's a lot of room to run in that. My employer has had users show a fair bit of interest in a modern experience with LDAP layered on top ( https://github.com/FusionAuth/fusionauth-issues/issues/954 ) and I talked to someone at a conference that had built a whole business out of virtual LDAP: https://www.radiantlogic.com . They were working with companies with multiple LDAP based auth systems, and providing a way to have apps see one view of the user.
Maybe kanidm isn't that project, but it seems like a modern OSS LDAP implementation would be welcomed by the software community.
Disclosure: I work at FusionAuth.