My favorite theory for why Linux got a head start is in this (long) comment I found some time ago here: https://news.ycombinator.com/item?id=21420338
Some excerpts:
"With Linux, I just booted from a Linux boot floppy with my Linux install CD in the CD-ROM drive, and ran the installation. With BSD...it could not find the drive because I had an IDE CD-ROM and it only supported SCSI."
"It insisted on being given a disk upon which it could completely repartition. [...] Linux, on the other hand, was happy to come second after my existing DOS/Windows."
"By the time the BSD people realized they really should be supporting IDE CD-ROM and get along with prior DOS/Windows on the same disk, Linux was way ahead."