I'm waiting for Tornado to support Java 11+ before I jump on the bandwagon. It's getting closer[1], but it's still not production ready, from what I can tell.
I just like Java. I know it well and mainly don't have issues with the problems kotlin is trying to solve. The main benefits I see of kotlin are coroutines (coming to java with project loom) and nullable reference types (which isn't a deal breaker for me).
I also don't enjoy functional programming, I find that it quickly becomes unreadable and kotlin is a weird mix between FP and OOP.