It took decades to get UNIX's reliable enough to trust for day to day operation with labor estimates in hundreds of millions to billions of dollars. Three or so people got that done in a few years with MINIX 3 using micro-kernel architecture and self-healing services. QNX and INTEGRITY RTOS's have been doing that stuff for years in the field with less resources. And QNX-based Playbook was more reliable, responsive, and fast than iPad in its first incarnation. Before that, KeyKOS outperformed IBM's monolithic stuff on their mainframes with better security and full persistence of system state.

So, they worked better and keep working better. People just use monolithic kernels for some reason. Just like they took forever to get off of C for reliable, business applications. "Worse is better."