1. Performance.
2. A good reverse engineer can dump the classes and members offset and easily hack my game. (Using the script engine)
3. I like C++ more than high level languages.

1. The performance critical part is the engine.
2. This is not as serious a priority as: finishing something that somebody would actually want to reverse engineer.
3. But your planning to work on a higher level! You program on top of the (low level) engine. Really the whole point of an engine is to let you use scripting.