Since Kotlin is made by Jetbrains (the developers of IntelliJ IDEA) so it stands to reason that the IntelliJ support for Kotlin is also great.
Since it runs on the JVM, Kotlin is 100% interoperable with Java and can use existing Java frameworks and libraries.
Because the standard library is pretty small and tight, it mostly consists of focused extensions of the Java standard library.
Kotlin's syntax is extremely easy to understand. The language can be picked up in a few hours just by reading the language reference.
Since Kotlin does not enforce any particular paradigms and is not purely functional, it can be pretty easy to fall back to imperative programming habits if a programmer comes from an imperative background.