They don't usually "Fork" much of kernel, they (mostly) just have drivers(modules) outside it. When refactors happen to kernel internals, they don't happen to external modules so it is hard to get them up to date with the latest version. Over time this gets worse and worse.

That said there are often some small changes to the kernel itself, which are also an issue. This is more common if the hardware implements something new that hasn't really been standardized in the kernel yet.