The main reason is that the java game can be played on Windows,Linux and MacOSX. Each platform has a lot of users, this is very important. Java is convenient for development on Windows,Linux and MacOSX.
Another reason is that both the server back-end and the front-end using Java language is more convenient. Using Java language is convenient for building a team, as well as facilitating internal communication and mutual help within the team.
Review after game finished, I think choosing Java is right, the goal of being able to play my game on Windows/Linux/MacOSX platform is achieved. Both the server back-end and the front-end using Java language can save a lot of time and energy. Especially, jlink which is introduced from java9, is really convenient for distributing games.
Video: My Java Game And Engine on Windows
Video: My Java Game And Engine on MacOSX
Video: My Java Game And Engine on Linux