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 types
Arrays in Kotlin are invariant
Kotlin 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 exceptions
Primitive types that are not classes
Static members
Non-private fields
Wildcard-types

What Kotlin has that Java does not
Lambda expressions+Inline functions = performant custom control structures
Extension functions
Null-safety
Smart casts
String templates
Properties
Primary constructors
First-class delegation
Type inference for variable and property types
Singletons
Declaration-site variance & Type projections
Range expressions
Operator overloading
Companion objects
Data classes
Separate interfaces for read-only and mutable collections
Coroutines

Comparison to Java - Kotlin Programming Language

footnotes:
Kotlin vs. Java: First Impressions Using Kotlin | ArcTouch
Java vs. Kotlin: Should You Be Using Kotlin for Android Development?
10 Features I Wish Java Would Steal From the Kotlin Language
Java vs. Kotlin: First Impressions Using Kotlin for a Commercial Android Project - DZone Mobile
Java 8 vs Kotlin
Does Kotlin have better functional programming support than Java 8 streams and lambdas?
Functional programming in Java 8, Kotlin and Scala
Java vs. Kotlin: Android Programming Languages
What are the advantages of Kotlin over Java?
Kotlin vs Java: key differences between Android's officially-supported languages
Kotlin Programming Language