It's a combination of things. Java is slow moving by the very nature of the contributors to the language and runtime favouring slow methodical advancement.

Additionally Java has a longer history than most of backwards compatible software that contributes to a higher burden of testing before making changes to the core language.

Finally there is the community itself which favours high performance and reliability and prefers where possible to make use of libraries rather than change the language or runtime itself.

That isn't to say Java isn't evolving or that the JVM itself is stagnant.

The introduction of the Java module system allowed the JRE itself to become a much lighter dependency. Things like Project Loom seek to bring Erlang-like fiber/green thread support to the JVM and will probably land in a release soon.

Also the release cadence has been changed so new releases are now landing much more frequently which is increasing the pace of development.