I have two good reasons not to choose Kotlin.  The first is until I started reading this thread on HN I had never heard of Kotlin.  The second is I attempted to find kotlin in a job listing on Monster.com in several major cities and managed to only find one single listing in Los Angeles.  The job listing says they are transitioning to Java.  My argument of course is from the greater view of the business and not a laser focus on Kotlin as a technical decision, but for <i>most</i> projects you should choose a much safer option with insane amounts of support and developers on the market by the boat loads.