Actually, there could be an excellent reason not to designate the lone candidate as a principal. Most Congressmen do not like to get sucked into the game of giving favors for contributions, believe it or not (they prefer the money with no strings attached). By continually staying with the unranked slate method, they pass the appointment buck, so to speak, to the Academy. In that case they can always say "The Academy made the choice not me" and cannot be accused of "selling an appointment" to a big contributors kid. This method works only if you do it year in and year out no matter who applys.
There is a flip side to this coin (that has nothing to do with the original question). On occasion, an Academy has offered an appointment to one of the 10 unranked nominees and the nominee turned it down for one of the usual reasons. Common sense would say that the Academy should go back to the original list of (now) 9 nominees and pick another. BUT it is not a legal requirement and sometimes the Academy picks no one from that Congressman's list and fills that slot in the class using another type of nomination. Needless to say, that pisses off a lot of Congressional staffs and Congressmen due to the work put into the screening and interviews and the fact no constituent's kid goes yet it is a taxpayer-funded school. For the next round, they subtly say to the offending Academy "If you don't like us giving you the choice of candidates, then we will pick the appointment with the principal and ranked alternates method. Screw you, Jack." That was almost verbatim what I heard from a staffer and good for them.