Java is not ideal for everything or everyone

While Java recently received some serious functional-programming-like features in version 8, at its core, it is still a statically typed imperative language. Not all developers prefer this programming style. Programmers coming from Python or Ruby may cringe when they have to write code for a fully typed language. This can be a valid reason for teams to adopt a different language than Java for JVM development.

Also, some problems can be solved much more elegantly when using a dynamic programming language, while for projects that require complex concurrency situations, a functional programming style is often more suitable. Finally, some libraries and frameworks simply feel more natural when used with certain languages.