Because Microsoft could've promoted it more but it didn't.

>What are the F#'s downsides which prevent its more widespread usage?

The two biggest downsides I've encountered are not related to the language itself. One is lagging support in Visual Studio (didn't support folders for long time, poor cross-language C#/F# navigation, no support in .NET Core projects etc.) and the other is the lack of examples/frameworks for WPF applications when I needed them.

Other than that it's my favorite language for all kinds of tasks -- from production apps to scripts.