Around the time Linux was starting to make inroads into businesses. Usually by the backdoor - internal services needed platforms they could install without going through the usual acquisition processes. If it cost money, some beancounter was going to object. (I witnessed a couple of cases first hand.)

So the techies installed some flavour of Linux on decommissioned boxes instead and ran their systems where they could. Individually the cost savings were negligible, but when you suddenly find out that 30 or even 50 internal services are running on top of OS stack that had cost nothing, the upper echelons finally take notice. The fact that the OS could be customised and repurposed for a particular need was an amplifier, and only later became the core "feature".

I recall (with horror) the days when "tweaking the OS" meant running regedit.exe and turning a number of undocumented knobs in directions you could only guess at. Not to mention pray that the system worked on the next boot.

So Linux use began as a way to cut costs (or skip the acquisition process altogether). It ballooned because it was possible to customise and extend the system on the OS level as needed.

• Linux worked. It was "good enough". Once you reached the point of running a shell, GNU userland, an editor, and a compiler, it was off to the races. Not sufficient, but necessary.

• No lawsuit. The UNIX Wars put a multi-year crimp on FreeBSD. The BSD splits didn't help much either (these included not just the userland as in Linux distros, but kernel development --which for Linux distros remains unified).

• Freely hackable, but published changes are licensed under GPL. I don't know how much thought went into Linus's choice of license, but it proved highly fortuitous.

• The time was absolutely ripe. Even within proprietary Unix companies, people were advocating heavily for a freely-available, open-source operating system. See Larry McVoy's "The Sourceware Operating System Proposal", from his days as a SunOS engineer prior to founding BitMover: http://www.landley.net/history/mirror/unix/srcos.html

• Key early adopters and developers. Alan Cox on development, and John "Maddog" Hall at DEC were key in both pushing kernel development, facilitating early ports (to Alpha), and in putting legitimacy behind the effort.

• Multiple distros. Almost immediately a side-light of bundling up the kernel and a set of utilities into a usable package emerged: Boot Root, MCC, TAMU, SLS, Yggdrasil. I remember SLS ("soft landing systems") being explained, somewhat fondly, as "because it was a pile of shit". Eventually over a few months in 1994 Slackware, Red Hat, Debian, SuSE, and Caldera emerged, which with very few notable exceptions continue to rule the distro landscape. I find interesting that "bootable distros" were present from very nearly the start (my own first exposure to these was Linux Root Boot, a/k/a Lnxrtbt, on a single floppy, also Trinux, booting from 2+ disks into a ramdisk). LWN has a good early history: https://lwn.net/Articles/91371/

• As others have noted, functionality, cost, features, and exigency all meant that technical needs felt at the coalface by front-line engineering staff at many firms were being met with decommissioned server, or, very often, desktop systems. While Microsoft-bound CIOs were proclaiming "we run no Linux and never will" their organizations were in fact increasingly not only using, but dependent on it. These denials persisted through the early 2000s, though beyond 1998 or so it was largely only dead-enders maintaining this.

And from there it just continued to build. I'd argue that the linchpin argument wasn't money made selling Linux or Linux services, but companies based on it which simply couldn't exist by other means, of which the most notable early exemplar was almost certainly Google. Running its systems on proprietary Unix or Microsoft platforms would have been prohibitive in both cost and flexibility.