Back when the choice was made, micro-kernels was all the rage in both academia and commercial ventures, and Stallman chose Mach since he thought it would speed

Back when the choice was made, micro-kernels was all the rage in both academia and commercial ventures, and Stallman chose Mach since he thought it would speed up development, he was hardly alone in choosing Mach at this time, Apple (MkLinux, NeXTSTEP), IBM (Workplace OS) amongst others. He fully acknowledged that he made a mistake in going with Mach and as soon as Linux took off FSF focused on providing the necessary software to combine with Linux into an operating system and placed Hurd on 'life support', where it's been ever since.