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.

Yuri