For needle in a haystack you have to pay attention to the thing that you are trying to find. Attention can do this pretty well.
When looking for an omission, that omission can be anything, you can only reason about it by comparing one whole context to another whole context. The attention layers can't really do that.
This is similar to the "rank a long set of things" problem. Absent some meta cognition process, they just can't do that.