Java was stagnant and ripe for being kicked off the top.
Scala was the hack that showed why Java was starting to suck but Scala suffered from no direction. Every single idea a PhD ever had was implemented in the language with no thought other than it seems cool to be able to do that too. Which is why all the Scala codebases fell apart, because you could write anything, anyway you want, and it was impossible to maintain without extremely strict guidelines on what parts of the language you were allowed to use. Also the build times were atrocious.
Kotlin designers evaluated language features across the ecosystem and chose what made sense to make Java better when it came out and now has easily surpassed Java. They are still thoughtful in what they choose to add to the language and it is now very powerful. However they smartly imitate Python by having clear guidelines on what choices you should be making when writing code so that other developers can easily dive in.