I have no special insight, but I know of 2 possible factors:

* the NetBSD "rump" unikernel is out there:

https://rumpkernel.github.io/

https://thamizhelango.medium.com/netbsds-rump-kernel-framewo...

* Secondly, this exists because NetBSD is much much smaller and simpler than Linux, which has undergone decades of enterprise-focussed optimisation.