A list of reasons that the Supreme Court Fellows Program gets very little attention , despite being, in numerical terms, far more selective than Supreme Court clerkships (a couple dozen clerks every term, but only four fellows):
1) Fellows don't get to write opinions. Clerks hold the power of Grayskull.
2) Fellows have multidisciplinary experience, with significant careers already under their belts. Clerks are twentysomething law geeks. Youth beats age.
3) Each branch of the federal government has fellows. Only the justices have clerks. They special.
4) Despite the name of the program, fellows are spread out, with each assigned to a different aspect of the judiciary: Sentencing Commission, Administrative Office of the U.S. Courts, FJC and SCOTUS itself. Clerks hang out together and can form friendships and resentments to last a lifetime.
5) Relatedly, fellows seem to spend most of their time with people other than the justices. Clerks are excellent sources for tell-alls.
6) Fellows don't have to hold JDs, and academics are favored; they aren't likely to become big names in the law. On a list of past fellows, David M. O'Brien was the only name I recognized, and that was solely because I took two classes with him. Supreme Court clerks have more than the advantage of superior numbers (certainly there are plenty of little-known individuals who have been clerks); clerking is either a near-prerequisite or just very strongly correlated to some positions, including being a federal appellate judge oneself, or a famous scholar or appellate advocate.
7) You can clerk, but you can't fellow.
 If the proposition that the Fellows Program is less well known requires support, I refer the skeptic to question 4 of the Fellows application: "How Did You Learn About the Supreme Court Fellows Program? In the text box below, please tell us how you learned about the Supreme Court Fellows Program." I am guessing this question doesn't come up in applying to be a Supreme Court clerk.
 Which makes each justice the Sorceress to her clerks, and the opponents of judicial review Skeletor.