Does it matter why? They asked their devs and got an answer. Kotlin is not just another cool language, it's now the main Android language. Staying with Java if you're on Android definitely should feel like staying behind... I only write backend Kotlin code, lots of it, and we have freedom to choose Java or Kotlin, the code base is mixed. Nearly everyone uses exclusively Kotlin for new code. Sometimes they go to the trouble of even converting some existing Java code to Kotlin before working on it. If you need to ask why devs prefer Kotlin, you probably don't have any experience with the languages.