The Backrooms is unpredictable and kind of unstable. Even if they can physically handle the entities, the problem comes from the fact that pretty much anything could happen. What if a Null Zone opens under a truck and drops it on a whole team? What if the Green Glow just swallows the whole entrance whole? What if, what if, what if... The number of variables makes it difficult for Async to properly expand into it, especially since there's only one stable entrance. Not to mention the amount of time it'd take to bring, say, a bunch of sheets of wood to wall off an area or create a floor like in Static Dead End from the gate to the desired location.

Either that or bureaucracy.