The LF program is open to rising second-year MBAs. Those who are selected to be LFs in their second year are often folks who have demonstrated strong social skills and good team/peer leadership ability; they are also especially interested in developing their leadership skills as mentors to the first-years. As one goes through the first-year, one gets a better appreciation for what kind of qualities are common amongst the second-year LFs.
Another important factor is that, for most LFs, being a Leadership Fellow will be the one - or one of the two main things they undertake at Wharton. It is a large time commitment, and those busy with several other projects, classes, and/or clubs will usually not be able to meet the time requirements of the LF role and position.