A Mono-related thread, it's been some time since we've had one of those! Oh dear, let's see what kind of crazies will show up this time.
Another reason: Mono provides one of the best (if not *the* best) scripting solution today for games. It's trivial to embed, it's fast, simple, interoperates with native code marvelously and has great IDEs, debuggers and profilers.
Third, this "alternative Java implementation" is one of the most pleasant programming languages available today. The only one that comes close is Python but that's a wholly different paradigm.