Visual Studio is not a factor here, value types, delegates (aka safe function pointers), mono SIMD and C# are the factors that mono was chosen over Java. C# is more expressive and concise than Java, C# supports unsafe (pointer) operations inside unsafe blocks which is very valuable for a game engine if such a thing is required. Mono has a more powerful eventing model through the use of delegates, where the Java equivalent of using anonymous inner classes is much more syntax and runtime heavy. C# supports semantic closures and has iterators, very useful features for scripting support.
Plus Mono is more open than Java, was developed by open source community, therefore was already more easy to modify than Java, which has in house developed source.
Plus Mono is more open than Java, was developed by open source community, therefore was already more easy to modify than Java, which has in house developed source.
My 2c.