The reason for this is twofold:
- Java 7 language features allow me to write less code for the same purpose. This helps in maintaining and improving the game.
- The official Oracle support for Java 6 has ended, which might leave your system vulnerable to Applet based exploits, not to mention the performance drawbacks.
The upcoming version 0.96 with the multiplayer will use Java 7 only. My expected delivery time is about 1 month from now. The change will affect the Launcher as well. Since Java 6 won't run Java 7 class files, I'll add a small Java 6 bootstrap code that warns the user about the version problem.