Because you can use the same toolchains, APIs, libraries, and even progressively migrate with both languages in the same project. Honestly, people who whine about Java 8 in Android and refuse to consider Kotlin are suffering by choice.
Only those that can be desuagared into Android Java, everything else will fail to either compile or map into ART features.
All of Kotlin runs fine on Android, that's its point, that all of it can be desugared to run on a 1.6 or 1.8 JVM?
If you don't want to use modern Java libraries as third party depedencies then all good.
Now if you plan to dynamically link to Java libraries that require JVM opcodes without counterparts on DEX, SIMD, Panama, Java Standard Library post Java 11, then thought luck.
See it as an opportunity to learn how to use the NDK (for SIMD), or write your own Kotlin versions.
At that point one could also say "why Anrdoid?".