For one thing, writing things in a microkernel are far more complex. In theory the isolation can mean lower impact of bugs and security issues; but in practice

For one thing, writing things in a microkernel are far more complex. In theory the isolation can mean lower impact of bugs and security issues; but in practice, the complexity means more bugs which are harder to debug. That's why you don't really see microkernels outside of embedded use cases -- the lower functionality requirement makes the complexity manageable. You can write a rocket OS as a "proper" microkernel, but there's no way you could write a full-blown Windows OS as a "proper" microkernel. That said, Windows or Linux can take leaves out of the microkernel playbook and apply them judiciously: KVM's architecture is arguably an example of this, where the core kernel does only the minimum necessary, and everything else is pushed to the user-mode component (QEMU, Firecracker, etc).