I think one of the limiting factors is Android, which is still stuck with Java 8. So if you're developing a library, which in theory may be used in Android app by others, you have to stick to Java 8 to make it compatible.
Re:Reasons to upgrade from Java 8
by mikemybytes on Thursday April 15, 2021 @04:15PM
(#26825589)
I guess that's why Kotlin became so popular in the Android world - all the new & shiny features are present but you still can target older bytecode versions
Google is the one to blame, they let Android Java stagnate on purpose, after all it helps to sell Kotlin.
Kotlin is free though, and a JetBrains product, not a Google one.
Kotlin is married to Android, it doesn't go anywhere unless JetBrains settles for a divorce.
Actually, they've done a pretty decent job on multiplatforming it. Can use it for servers, compile to JS, even compile through LLVM and target iOS if that's your jam. Android is the killer use case, of course, but I think it'll survive losing preferred status on Android.
I investigated creating a library for Java recently, and could have made a pretty decent API for Java 9+, but with so many people on Java 8 and below, it might not have been worth it.