Some reasons that spring to mind:

- Compile speed - Kotlin is still painfully slow - about 4-5 times slower than equivalent Java functionality (rough estimate)

- Kotlin is completely proprietary, no community or openness, no alternative to Jetbrain's IDE

- Kotlin (for me) has passed from being a simpler/more terse/attractive version of Java into being a complex language with lots of strange corner cases. Particularly largish Kotlin code-bases can be hell to read as Kotlin supports and encourages a DSL like approach.