Don’t get me wrong F# seems like a nice lang but definitely not mainstream (non geeks don’t use purely functional langs).

You can’t deny Kotlin is bigger lang than F# by any ranking from those popular (Tiobe) to unknown ones and is build on top of much bigger ecosystem (JVM) - that’s IMHO much more important factor for anyone doing commercial projects.

Additionally there is a big incentive to use it over Java (if you are stuck with older JDK) contrary to using F# over C#

Kotlin as a lang is better than C# (which is normal - it’s newer) and has better default IDE (IntelliJ) than anything on the market.