Because we know that for a given project budget we can have a human annotator review a limited number of instances, we are interested not so much in which model is the best at identifying positive instances across a full test-set, but rather which model will sequence its output in such a way as to return the largest number of true positive predictions within the top n instances ranked by the model as positive.
accuracy is not an ideal measure in this situation, and relying exclusively on it can have very expensive consequences.