A microkernel is has more processes, and those processes have their own state and their own MMU configuration. They have to pass data between themselves to com

A microkernel is has more processes, and those processes have their own state and their own MMU configuration. They have to pass data between themselves to communicate by exposing data and mapping memory regions/structures across, which otherwise could be a function call with a pointer. A monolithic kernel is either running or panicked, with either valid or invalid memory. The microkernel adds more failure modes - such as previously always present parts of the system going away, resetting their internal state, hanging, being upgraded on a live system, and so on. The costs of the protections and of IPC recovery are what push towards monolithic designs. Modern microkernels have had a lot of focus on those two areas - often being little more than memory management and IPC at the core. I'd say even though new microkernel systems haven't displaced the current ones, research on these newer "nano kernels" have borne fruit in various hypervisors.