Java 8 is a mature language and with more developers and a community to support its use in android development with great results and productivity and more developers are used to Java 8.
Kotlin is safe, concise, interoperable, tool friendly and comes with data classes and other useful features below which java 8 does not offer.
“Kotlin has way, way more to offer than Java 8. Here is just a sampling (in no particular order)
Null-safety
Type inference
String interpolation
Data classes
Delegation
Smart casts
Conditionals as expressions
Sealed classes
The when expression
Extension functions
Co-routines in 1.1
... and I'm sure I've just scratched the surface”-What if the future of Kotlin when/if Java 8 becomes available on Android? • r/Kotlin
How will Java 8 impact Kotlin?“Comparison to Java Programming Language
Some Java issues addressed in Kotlin
Kotlin fixes a series of issues that Java suffers from
Null references are controlled by the type system.
No raw typesArrays in Kotlin are invariantKotlin has proper function types, as opposed to Java's SAM-conversions
Use-site variance without wildcards
Kotlin does not have checked exceptions
What Java has that Kotlin does not
Checked exceptionsPrimitive types that are not classes
Static membersNon-private fieldsWildcard-types
What Kotlin has that Java does not
Lambda expressions+Inline functions = performant custom control structures
Extension functionsNull-safetySmart castsString templatesPropertiesPrimary constructorsFirst-class delegationType inference for variable and property typesSingletonsDeclaration-site variance & Type projectionsRange expressionsOperator overloadingCompanion objectsData classesSeparate interfaces for read-only and mutable collectionsCoroutines
Comparison to Java - Kotlin Programming Language
footnotes:
Kotlin vs. Java: First Impressions Using Kotlin | ArcTouchJava vs. Kotlin: Should You Be Using Kotlin for Android Development?10 Features I Wish Java Would Steal From the Kotlin LanguageJava vs. Kotlin: First Impressions Using Kotlin for a Commercial Android Project - DZone MobileJava 8 vs KotlinDoes Kotlin have better functional programming support than Java 8 streams and lambdas?Functional programming in Java 8, Kotlin and ScalaJava vs. Kotlin: Android Programming LanguagesWhat are the advantages of Kotlin over Java?Kotlin vs Java: key differences between Android's officially-supported languagesKotlin Programming Language