The nebulae is frequently plagued by electrical and ion storms making subspace travel dangerous, The Guardians, listening posts positioned in the nebula to observe the hyperspace route required special shielding and being placed in breaks in the nebulae. And it was because of this that the Theralis and even Midgard Sector remained, mostly, untouched by the rest of the galaxy even after it became obvious something was going on behind the curtain.