It is closed source, isn't it? I assume it is, like ADOM, to keep the secrets. Though there is an obvious solution: you could separate the game data from the engine, and open-source the engine, for public benefit and participation.
There are no secrets to keep; it's not like an RPG where the ending can be spoiled. Well, not really any secrets, anyway. Anyone who lives in a country where Tolkien ever published a book will have some ideas about what to expect.
It's really just about creative control. He has let people help in limited ways in the past though. There were some improvements to the way it uses the OpenGL API and to the way it renders fonts (it can now use TrueType fonts for most purely-textual elements) that were contributed by a fan. he later stated that although it was a good improvement to the game, it wasn't an unalloyed good. The changes _worked_, but he no longer understood that part of the code very well.