Umm because they are dead space.
Anything that is--gated where you can't warp to the room directly--you can't use a MWD in.
You get into deadspace though an acceleration gate.
You can always warp out, but you, or anyone else, can't warp in.
Once you are done with a mission the dead space pocket disappears and you can warp straight to your wrecks to salvage them (as can ninja salvagers.)
There is, of course, an exception to the rule: faction warfare complexes: tactical or mission-based, allow MWDs.