Linus (impatient with the pace of HURD) developed a quick and dirty kernel that a Unix user land could be built on top of. He took a lot of shortcuts, he didn't think too much about portability and basically just made a beeline for the end line - to get a shell and hence other stuff running over a kernel. The kernel filled out and became portable as the project gained momentum and volunteers.
Whereas HURD got stuck up its own ass for correctness and politics. And that's even before Linux existed as a thing. It's hardly a surprise that when Linux did appear that people jumped ship.
It's true there was a debate about micro kernel designs but that alone doesn't explain HURD's failure.