Kotlin is fatter, compiler is slower, code completion is slow as hell on large projects, but other than building small applications - there's really no reason to not use kotlin except for the fact that you need to actually learn the language or else you're going to end up with very very slow codebase where opening a file and waiting for syntax highlighting takes 2-3 seconds and typing autocomplete is just painfully slow.
"fatter, compiler is slower, code completion is slow as hell" - if that's all you want out of your programming language, then Java is probably a good choice for you.
For others that value the things that Kotlin brings over Java (even modern Java), and for the ways in which it delivers a simpler experience than Scala - I think it's a pragmatic and sensible decision.