As a simple explanation.
Depth - Away from Unit (Minor, Major servicing / upgrade etc)
Forward - Yes available for Ops BUT every now & again maintenance prevents their use (Fuel leaks that take weeks to fix, OOPs maint etc). As good as us Engineers are we sometimes run out of quick fix spray.
Or
Depth - Making money for our supporting arms.
Forward - Overused / stretched beyond limits / extended beyond belief / skill fade preventing expedient return to service!