Objective-C is nothing but a thin layer(a bit thicker with 2.0) of syntactic sugar for message passing on top of standard C. Even the most basic object orientation is provided by the runtime library, which was proprietary for a long time. Inertia is an important factor in language use.
It shines especially on GUIs, but the only toolkits that support it are Apple's and the mostly unknown and catchup-playing GNUStep.
While there is some value for Objective-C outside of GUIs, and I think people would use the extensions were they imported to C, even in system code, there is little reason to choose it over alternatives, when little of your system is meant to work with it.
