THIS. It's a BIG search area because things are further away from each other that far out and thus the gravity perturbations on the other planets and small bodies is small. Small effect means we only know the general area it's in. Couple that with the dim lighting that far away from the sun and it means you have to stare for a long time to see the dim objects, and thus it takes longer to search that part of the sky with enough sensitivity to see things this dim.