Backward compatibility with existing compilers.

And more importantly, not breaking old, old codebases, which is a very critical problem.

Obj-C++ and D don't have critical codebases, because there is just so much less code. Apple does what they want.

Like Stroustrup says, when a language is not used, it won't generate problem: I gues his quote is often quite overlooked, but it's very very true.

Compilers are a difficult topic.