Kotlin has strong commercial support from JetBrains, a company that delivers very popular IDEs for many popular programming languages (Android Studio is based on JetBrains IntelliJ IDEA). JetBrains wanted to improve the quality of their code and team performance, so they needed a language that would solve all the Java issues and provide seamless interoperability with Java. None of the other JVM languages meet those requirements, so JetBrains finally decided to create their own language and started the Kotlin project. Nowadays, Kotlin is used in their flagship products. Some use Kotlin together with Java while others are pure Kotlin products.
Kotlin is quite a mature language. In fact, its development started many years before Google announced official Android support (the first commit dates back to 2010-11-08):
Note
The initial name of the language was Jet. At some point, the JetBrains team decided to rename it to Kotlin. The name comes from Kotlin Island near St. Petersburg and is analogous to Java, which was named after an Indonesian island.
After the version 1.0 release in 2016, more and more companies started to support the Kotlin project. Gradle added support for Kotlin into building scripts; Square, the biggest creator of Android libraries posted that they strongly support Kotlin; and finally, Google announced official Kotlin support for the Android platform. This means that every tool that will be released by the Android team will be compatible not only with Java but also with Kotlin. Google and JetBrains have begun a partnership to create a nonprofit foundation for Kotlin, responsible for future language maintenance and development. All of this will greatly increase the number of companies that use Kotlin in their projects.
Kotlin is also similar to Apple's Swift programming language. In fact, such is the resemblance that some articles focus on differences, not similarities. Learning Kotlin will be very helpful for developers eager to develop applications for Android and iOS. There are also plans to port Kotlin to iOS (Kotlin/Native), so maybe we don't have to learn Swift after all. Full stack development is also possible in Kotlin, so we can develop server-side applications and frontend clients that share the same data model as mobile clients.