Earlier quoted context omitted.
It's because C# only "recently" became cross platform in a reasonable way. Hard to overstate the mindshare that it lost for that
And it's still not as cross-platform as Java is, or pretty much all other tech stacks. FreeBSD and commercial UNIXen are not supported at all. If you prefer to develop on macOS or Linux (hi there), you don't get access to many Windows-only tools (mostly for debugging and performance measurement). Microsoft only cares about other operating systems to inasmuch as you can host the resulting application there, but really…
The bias does exist but it's going away, especially that now a lot of developers are moving over to M-series macs.