Being in the .NET ecosystem exclusively can also mean a reliance on IDEs and .NET-specific tooling to be productive (or things like Eclipse refactoring for Java).
And there's the simple fact that there are a lot of qualified candidates who do more than just .NET/just Java. Polyglot programmers are more likely to be flexible and be in the right environment.
Of course people can be good enough, but it's not just about the looks. Things happen differently depending on the tooling.
Now, the extended part of this discussion is that maybe it's fine, and companies should be able to onboard people into a technology. Because, hey, it's just programming right? But that could mean looking at a multi-month training session until you're looking at someone as productive as the guy who's familiar with the tech already. Not all experience in programming is equivalent.
An aside: I feel like this discussion usually has a lot of "web programming is easy" undertones (it hasn't been explicit but we all know what "trendy" means). But I don't think there would be as much of this discussion if it were something like "web-only developer applies to work on embedded systems with basically no C experience".
"Programmers who used Java or C# (when interviewing with us) go on to pass interviews with companies at half the rate of programmers who use Ruby or JavaScript. (The C# pass rate is actually much lower than the Java pass rate, but the C# numbers are not yet significant by themselves.)"
Actually, it's Mac on the desktop, Linux on the server.
If the hiring managers at these companies see a resume with just ".NET/Winforms/WPF/ASP.NET", they perceive a negative signal.
Run, don't walk, from doomed companies like that.
I don't have evidence of a wide random sampling of "trendy" employers but based on anecdotes[1][2], I think it is.
Many trendy companies (a.k.a. "hot startups") overwhelmingly favor a Linux + open source stack. If the hiring managers at these companies see a resume with just ".NET/Winforms/WPF/ASP.NET", they perceive a negative signal.
Yes, writing a loop to display a list in ASP.NET should be Turing equivalent to writing a loop in Node.js/Golang but people don't think purely on equivalences of computer science concepts. They also think about "culture" and a resume that's exclusively ".NET" looks "corporate" and "enterprisey" instead of "cutting edge" and "hip".
My advice to potential job seekers who only have "MS .NET platform" as a skill: start a github profile with projects using Linux/Node/Golang/etc to help counteract the adverse cultural selection bias.
[1]https://blog.expensify.com/2011/03/25/ceo-friday-why-we-dont...
[2]"And the .NET developers are mostly enterprise folks who don’t like working in rough-and-tumble startup world. Remember, I worked at Visual Studio magazine and had a front-row seat." from http://scobleizer.com/myspaces-death-spiral-due-to-bets-on-l...