I think the main reason is poor marketing. Teams built microkernels, but of course a microkernel is not useful on its own so they had to build a complete OS. But being already tired (or near graduation) from building the kernel itself, they simply ported Unix to run on top of their microkernels, which led to a slower OS with no new features. There are benefits to microkernels, but all these benefits were hidden or wasted by Unix. By this time, patience (and thus funding) for microkernels had run out and researchers had to find something else to work on.
