Java and C# have 'managed' runtimes; it the language manages memory for you. This memory management bit cannot itself be written in the 'managed' language. Also, an operating system can't either[1].

Also, systems with hard real time requirements, such as audio/video, the signal processing code in your phone, wifi router, etc, are incompatible with almost all managed runtimes.

[1] Okay, I know there are academic ones that are. As far as I know, no productions systems are.