I don't think they're necessarily the only candidates for the awards, as the list can, and does, change as the season progresses. It seems like these are the guys who, based on last season, would be the most likely to win that award if they play exactly the same way or better during the current season. My theory is that the "candidate list" is an easy way for game announcers to decide/build up gametime storylines, and for the analysts to pick some kid in the beginning of the year and pat themselves on the back vigorously if he wins said award. ("I've been talking about this kid since Day One, that he had the potential to be a game breaker if he just blah blah blah." They just never mention they've said the same thing about every other kid on that list, and hope no one remembers back to August when it's December.
It's either that or a way for fans of teams who aren't contending for a national title anytime soon to be proud that they've got a Davey O'Brien, Doak Walker, Butkus, Ray Guy, etc., potential award-winner on the squad. :)
"You rock, Rolfski. You are one hep cat." "Ya, ya, sock it to me."