The reason is Google kept Android stuck on Java 6 syntax for so long that the community moved on. At the time, moving from Java to Kotlin was a huge deal and then Jetbrains made a good job in making the tools work flawlessly and with no performance penalties as everything is compiled to Java bytecode (besides the nice interoperability).

Now Java has been upgraded on Android but it was too late.

Kotlin is now preffered over Java by Google

https://developer.android.com/kotlin/first

You can still build latest apps using Java and traditional view system. There is still insurmountable of Java apps out there easily eclipsing Kotlin apps.

I prefer Kotlin because the language is so much nicer to write in. Also because the future is moving in this direction.

You can read this GTP summary on why Kotlin over Java. Or find some other dozen articles

::: spoiler summary

Kotlin has become increasingly popular for Android app development for several reasons:

1. Conciseness: Kotlin code is often more concise than equivalent Java code. It reduces boilerplate code, making development faster and less error-prone.

2. Null Safety: Kotlin's type system helps prevent null pointer exceptions, a common issue in Java. This improves app stability and reduces crashes.

3. Interoperability: Kotlin is fully interoperable with Java. You can use Java libraries in Kotlin projects and vice versa, making it easy to transition from Java to Kotlin.

4. Functional Programming: Kotlin supports functional programming constructs, which can lead to more elegant and maintainable code, especially for complex operations.

5. Extension Functions: Kotlin allows you to add new functions to existing classes without modifying their source code. This is useful for enhancing existing Android APIs.

6. Coroutines: Kotlin offers native support for asynchronous programming using coroutines. This simplifies managing background tasks, making your apps more responsive.

7. Default Arguments and Named Parameters: Kotlin allows you to specify default values for function parameters and use named parameters, enhancing code readability.

8. Smart Casts: Kotlin's type system automatically casts types when certain conditions are met, reducing the need for explicit casting in code.

9. Data Classes: Kotlin's data classes simplify the creation of classes meant primarily for storing data, reducing the amount of boilerplate code.

10. Great Tooling: Kotlin is well-supported by Android Studio and offers excellent tooling for Android app development.

While Kotlin offers many advantages, it's essential to consider your team's familiarity with the language and any existing Java codebases. Transitioning from Java to Kotlin can take some time, but many developers find it worthwhile for the benefits it brings to Android app development.

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Kotlin doesn't really offer a performance benefit over Java. It's negligible. It's mostly the enchanced type safety.

Kotlin being a requirement really depends on job ur applying for. There is still full Java teams. But imo more are switching to Kotlin. Or taking sorta hybrid approach. Imo Kotlin is great and the only downside i have is that learning it will make you feel annoyed when writing Bloated java again.