1. Unix vendors were tearing each other apart in the Unix Wars.
2. Linux was cheaper than Windows NT.
The second point is critical: many industries had Unix-based software and even the source code of said software, they just needed a platform to run it on. Linux on x86 servers was orders of magnitude cheaper than Solaris on SPARC, HP-UX on PA-RISC, or AIX on POWER. And for the existing SPARC/PA-RISC/POWER servers running these UNIX OSes, the investment wasn't entirely lost because they could put Linux on those too.
That is what made Linux on servers win. It wasn't better. It was cheaper.
We have spent most of the 2000s catching up to Solaris, AIX, and other server Unix systems. The 2010s was all about catching up to desktop Unix systems like IRIX, Solaris (again!), and NeXT.
Now? We're at the tail end of that effort. Who knows what's next. :)
Cheaper is better, all other things equal. If the other systems were ten times more expensive, were they also ten times better in some other metrics to compensate?
And it is funny 20 years later there are people who still dont understand why Linux failed to takeover desktop.
I will quote Benedict Evans [1] who seems to be the only few with a decent understanding of Tech and Business.
>"the ideology of (one extremely narrow concept of) freedom works very well for small groups of true believers, but people didn’t move servers to Linux because of freedom - they adopted it because it was a better product."
[1] https://twitter.com/benedictevans/status/1425904537727086594