IMO Linux won because its license forced everyone who used and extended it to open their changes - this changes the calculus for firms building products on Linux to make it more worthwhile to upstream their changes to reduce their local maintenance burden.
Compare this to something like the playstation 4, which used freeBSD as the base of their OS, and contributed nothing back to the project at all.
Linux "won" because SGI very loudly shit the bed and betting on Itanium, taking IRIX with it, DEC was in the process of making the future of Tru64 very confusing with the Alpha, a computer that didn't support their flagship OS, and HP was EOLing HP/UX because they bet everything on IBM's OS/2 Warp.
The only big UNIX vendor left was Sun Microsystems, and Solaris indeed dominated the 90s dotcom era. Everybody was running SPARC and SunOS servers.
It wouldn't be until the mid 2000s when Linux started picking up the pieces left behind after Red Hat started their server product and certification program.
For a long time, Linux was strictly a hobbyist OS. It later dominated by simply being the last one standing after everyone else fell.
The true competitive threat to the Unix vendors wasn't each other, it was Microsoft.
And "for a long time" was actually a fairly short time. Linux began to approach feature-comparability fast, and ran on PCs, not $10,000 workstations (that were getting beat power-wise by PCs).
This seems a bit of a chicken and egg problem... why would the early adopters have chosen Linux, before others had been forced to contribute back their changes? The first company to adopt it wouldn't have received any benefits, only an obligation. Why pick it over BSD?
I think there were likely other factors that made it win out.
They would also get the "promise" that the system they were betting on would also get contributions from other companies, making a safer long term bet.
No. AT&T created Unix but was unable to market it due to a previous antitrust action. So they gave it away. (They required a license signature (I've signed that! :-)), but did not charge and were very lenient.)
The UCBerkeley Computer Science Research Group (if I've got my acronyms right) was one of the recipients and went on to add on many, many features and releasing the result under the BSD license.
Many people built companies like Sun around selling BSD Unix, including many alumni of UCB.
Then AT&T got out from under the consent agreement and began selling its own Unix, System V. By this time, Unix was a major player in the workstation market (a market that has largely disappeared as PCs got more powerful).
By, say, the mid 1980s, there were many, many companies selling many, many varieties of Unix, all descended from the original Unix via BSD or via BSD+System V. Most of them had some unique, valuable features (Irix's graphics, AIX's LVM and journaling file system, etc.) and all of them had modifications to lock customers into their version. This is where the POSIX standards come from (second only to ecmascript market manipulation goofiness), and why things like Autoconf/automake and the much-loved imake (not really) exist. There was much in-fighting; Sun vs everybody else, everybody else vs. IBM, etc.
Then two things happened: PCs got more powerful and began eating into the bottom of the workstation market (PCs ran DOS+Windows, which was and arguably still is, technically inferior to multi-user-by-design systems like Unix[1].) And PCs got more powerful and began to be able to run more advanced OSs (think "memory management").
At this point, the Unix world began to conflict with the Windows world. Unix was technically superior, Windows had more public and developer mind-share. But the Unix world was still more interested in fighting each other and stapled all of their arms and legs to that particular tree.
The end result was that Windows became and remains the most-used operating system[2]. All (almost) of the commercial Unixs died (almost; there's still some animated corpses around)[3]. The two counter-examples are MacOS, which is completely locked to Apple hardware, and Linux.
Linux is the interesting case. Windows and commercial Unix all had a 15- to 20-year head start. But Linux achieved (mostly) feature parity quickly and did not break down into multiple, competing streams. Both of those are due to the GPL; you can fork GPL software all you want, but you cannot add a feature to a fork and expect it not to be back-ported into the original if it's useful. You also have a very hard time locking users into your fork.
The bottom line is that Microsoft won the Unix wars, because the Unix licenses allowed companies to take Unix proprietary.
[1] Modern Windows is kinda-sorta based on VMS, another workstation OS, but not really and then they walked that back, and so on....
[2] I don't really consider Android or iOS to be general-purpose OSs. And they're both rather their own little islands, no matter how much the underlying tech shares with the rest of the universe.
[3] The Free/Net/Open/DragonFly BSDs are, I'm sorry to say, noise. And did you notice that I had to mention four of them?
