Linux isn't written with the use case of using bits of it in isolation in mind. Sure, you can do that, but the Linux developers don't have making that easy as a design goal, so it will often be quite hard, and even when it isn't so hard that is just an accident. By contrast, the OSKit developers had as an explicit design goal to make it easy to use bits of their code in isolation from the rest.

Linux gives no API stability guarantees for its internal APIs, only for the system call interface. By contrast, the OSKit developers intended the API of each component to be a stable documented interface. The Linux developers don't want Linux's internal APIs to be stable documented interfaces because that has a cost in that it slows down refactoring, which in turn reduces the various benefits that refactoring can produce (better performance, better maintainability, new features, etc)