Nitpick/clarification: it currently supervises the security posture, attestation state and overall health of several billion(?) Intel CPUs as the kernel used by the latest version of the Management Engine.
If ME is shut down completely apparently the CPU switches off within 20 minutes. Presumably this applies across the full uptime of the processor, and not just immediately after boot, and iff this is the case... percentage of Intel CPUs that randomly switch off === instability/unreliability of Minix in a tightly controlled industrial setting.
I used to occasionally boot into QNX on my desktop in college. It was a very responsive and stable system.
Hypervisors are, to a first approximation, microkernels with a hardware-like interface. All of this kernel bypass work being done by RDBMSes, ScyllaDB, HFTs, etc. is, to a first approximation, making a monolithic kernel act a bit like a microkernel.
Why something hasn't been done is always a hard question to answer, since to succeed a lot of things have to go right, and by default none of them do. But one thing is that microkernels were more trendy in the 90s - r&d people are mostly doing things like "the cluster is the computer", unikernel, exokernel, rump kernel, embedded (eg tock), remote attestation since then (I'm not up to date on the latest).
You might be referring to the previous versions. Minix 3 is basically a different OS, it's more than an educational tool - in fact it's probably running inside your computer right now if you have an Intel CPU (it runs Intel's ME chip - for better or worse).
One advantage of microkernels is that the "watcher" is so small that it could be run directly from ROM, instead of loaded into RAM. QNX has advocated that route for robotics and such in the past.
Minix may not be the best example of the type. While it is a microkernel, it's real world reliability has been poor in the past. More mature microkernel operating systems like QNX and OpenVMS are better examples.