Because it's not FLOSS. At least that's my reason.
Because I absolutely love Kotlin and I feel I'm much more productive with it. I have a hard time working with script languages and feel confident on my code nowadays.
Also, Kotlin let you use tons of java libraries.
C# is hardly a script language, also Kotlin Native is hardly a tool for gaming with its lack of real multithreading support
I both wrote games with Unity and Kotlin. Kotlin makes me much more productive and libgdx is well more suited for 2D games. Backend in unity?