One neglected moment in Linux history was the arrival of the Pentium II processor with Deschutes core in 1998. Intel had been making capable 32-bit processors since the 80486, but these processors were handily outperfomed by Alpha, MIPS and SPARC. The Pentium II 450MHz turned the tables. These high-end PCs easily outperformed the MIPS- and SPARC-based workstations and drew level with the much more expensive Alpha.
UNIX™ users looking to update their expensive workstations looked at a high-end PC and thought "I wonder if that runs Unix?". Inserting a Red Hat Linux 6.0 CD into the drive slot and installing the OS lead to the discovery of a capable and mature operating system, a better Unix than the UNIX™ they had been using previously. With a few years the majority of UNIX™ systems administrators were familiar with Linux, because they were running it on their own workstations, whatever Unixen they were administering over SSH.
This familiarity in turn lead to an appreciation for Linux's stablity. When it was time to field new small services — such as DNS and DHCP — then it was financially attractive to serve these from a Linux platform rather than a UNIX™ platform. Moreover the Linux distributors did a much better job of packaging the software which people used, whereas the traditional Unix manufacturers took a "not invented here" attitude: shipping very old versions of software such as DNS servers, and making users download and compile simple tools rather than having rhe tools pre-packaged for simple installation.
The Linux distributors did such a good job that it was much easier to run a web site from Linux than from Windows. The relative importance of these 'Internet' applications was missed by a Microsoft keen to dominate the 'enterprise' market. Before 1999 the ambition of Microsoft to crush the Unixen looked likely. After 2000 that ambition was unrealistic hubris.