Ignorance.

Bias against Microsoft. The usual self-righteous boycott of the evil Behemoth Corporate entity. And yes, .NET technologies are now more open-source. Visual Studio Code runs on all OS platforms. Visual Studio is set to be released on the MacOS, and on Linux distros subsequently. F# is an open source functional language, .NET Core also has been open-sourced. And yes, with Xamarin, you can build native Android or iOS apps. Most of the hate against Microsoft boils down to bias against the corporation.

Debo Ayangbile I presented those very facts, so why do you call me ignorant and biased? Until recently, Microsoft was Windows-only, just like Apple is macOS/iOS-only. However, a third-party company usually wants to reach a broader audience than just people on one OS. So they choose the cross-platform option, which C# was not. You cannot change things overnight. Add to that, that Java is the industrial standard and C# only a Microsoft product. Today, even though .NET is available on other platforms, Java codebases already exist and developers are proficient with Java. Is there any reason to rewrite all that code? Other than burning money, introduce new bugs, reduce features which need more time to implement and delay actual improvements, there is no effect. That's why developers didn't choose .NET and still go with Java.