Just as students seek out the most prestigious clerkships, judges seek out the best law clerks. And once a judge is known as a feeder, that reputation is self-reinforcing. Top students want to clerk for Judge Feeder because she has placed clerks with the Supremes, while the Supremes in turn come to regard an initial clerkship for Judge Feeder as a valuable credential for a potential Supreme Court clerk. Thus, the competition among judges for the top law students is keen. A top appellate judge is just as eager to hire a future Supreme Court clerk as the clerk is to become one.
Law students are notoriously risk-averse.
Thus, they are likely to take the first acceptable clerkship offer they receive, and students do not usually apply to any judge whom they consider unacceptable. Judges, of course, know this, and take advantage of it. It is easy to see where this all leads. The result is that any increase in quality of clerks for a particular judge in a given year is quickly erased. And as application and interview dates move further and further backwards, students' records become flimsier and flimsier, and the judges can be less and less confident each year that they have indeed identified the best students.