This is revisionist history. Linux eventually supplanted FreeBSD, which was widely used and popular in the 1990s. FreeBSD was also objectively the better OS back then, a clean and reliable system.
What made Linux successful over the long-term IMO is that they evolved the kernel much faster and earlier than FreeBSD, particularly when it came to hardware scalability and performance, which was advantageous with the progression of Moore’s Law. The cluster-style supercomputers were developed almost entirely on Linux as a result, which created a positive feedback cycle. Second, the Linux user-land quickly became really pleasant to use compared to most other UNIX flavors at the time because they weren’t too concerned with purity or being strictly interoperable with some other UNIX.
Most people in the industry today don’t remember the era of at least a dozen different UNIX-style OS being widely used. I probably used half of them in anger at some point. In that environment Linux won by having a superior UX out of the box for a lot of use cases, even if you paid the absurd money for one of the proprietary UNIX flavors. Later Linux started demonstrating better performance too. FreeBSD was definitely tidier and cleaner with its much slower evolution but sometimes worse is better.