The Linux kernel took off after Linus receive the MINIX book, a.k.a Operating System Design and Implementation by Tanenbaum, which include the source code of MINIX. Until the arrival of the book Linus was playing "Prince Of Persia" on his new 386AT desktop.
There were several microkernel-based research/hobby/free kernels in early 1990s, like L4, VSTa, or even HURD. Perhaps the reason why Linux succeeded as a general purpose OS (and not one of these) is because of its conservative design.