Very oversimplified answer: Microkernels still have a performance hit, end-users care more about performance than stability/security (at least at the ratio/marg

Very oversimplified answer: Microkernels still have a performance hit, end-users care more about performance than stability/security (at least at the ratio/margin in question), therefore we use the faster monolithic kernels for most user-visible systems, with carve outs where the performance hit isn't so bad (hence NT/Darwin really being more hybrid now, and even Linux can do some drivers in userspace). As other comments note, this only applies to places where performance matters; lots of embedded systems do in fact care more about reliability than performance so the outcome is different.