I'd say notch shipping his game early and updating the game frequently is what made the game incredibly successfull. I started playing the game when he was incrementally adding multiplayer functionality. He used to release a few features one at a time every week. I always looked forward to see how the game would improve next week. If he hadn't written the game in java modding support would turn from a merely hard problem to an impossible one. The first mods were written in obfuscated java code but minecrafts coders pack (MCP) enabled a mechanism to map obfuscated names to "reverse engineered" names which can then be applied to newer versions of minecraft. With C++ you could probably get away with manually decompiling points of interest which would easily take more than 10 times the effort and still create mods but as soon as the next update is released all that work will be for naught and you'll have to do everything over again whereas with java the decompilation is mostly automated.