Doesn't Dart+Flutter cover all those platforms as well? Also Delphi is 90% way there (still ways to go on wasm I believe). So not really "first and only"...
A lot of libraries don't exist in Dart or are platform specific (for example you can do SFTP only on Android and iOS, but not on desktop).
On the other hand, Flutter as a UI framework is much better than anything Microsoft makes (and they make a lot!). It's way more easy to build something beautiful with flutter.
Missing lib's are a big point, but we have to take the age of both languages, the size of their communities and the intended purpose into consideration.
C# was released around 2000 massively pushed by Microsoft with Desktop as primary target. Dart was released around 2010 or 2011 and was more or less intended to be a web language like JS/TS IIRC.