Most companies are writing software with software developed on Linux first and for Linux first (or Unix) and later ported to Windows as an after thought. I'm thinking Python, Ruby, NodeJS, Rust, Go, Java, PHP but not seeing as much of C#/ASP.NET which should at least be 20% of the market?
Only two explanations - either I am in a social bubble so don't have exposure or writing software for Windows is so much easy that it takes five times less engineering muscle.
That said, I'd guess the difference is that the startup and big tech world (i.e., "software companies") like our fancy stacks, but non-software companies prefer stability and familiarity. It makes way more sense for most companies to have a 3-man "bespoke software" department (sys/db admin, sr engineer, jr engineer) on a stack supported by a big company (Microsoft) where most of the work is maintenance and the position lasts an entire career. It's a big enough team to support most small to middling businesses, but not so big that the push to rewrite everything in [language/framework of the week] gains traction.
The practical conclusion is that these companies have few spots to fill, and they probably don't advertise where you're looking.
you clearly are.... There are TONS of windows only software out there, and most INTERNAL systems that run companies, these internal LOB apps, often custom made for the companies, many many many of them (probally more than 50%) are windows server apps.
For example GE makes a Huge Industrial ecosystem of applications that runs a ton of factories, utilities, and other companies... Guess what all of that is windows based.
Many of the biggest ERP's run on MS SQL Server which until very recently was Windows Only, and most MS SQL Servers are still on windows server
To claim only 20% of all workloads are Windows shows an extreme bubble most likely in the realm of WEB BASED DEVELOPMENT, as highlighted by list of web technologies, php, node, etc..