To the amateur cryptographer, or simply the budding enthusiast, the Beale Ciphers represent the ideal, a perfect challenge, and, the clincher, buried treasure. Starting as children, we are read pirate stories: buried treasure, untold riches, fame and fortune for those who find it. The fact that the Beale Ciphers have a story to go along with them increases the draw. The possibility that a story is all this is lends to the sense of mystery, and to the success if one were to succeed in deciphering the first and third ciphers. For amateurs, I think there is always a pull around trying to decipher the big ciphers that no professional has been able to crack. Part of that pull seems like a lofty dream, and part of it seems like human ego: the "I can do it, even though no one else can" mentality.
To professional cryptographers, though, the Beale Ciphers, while they have all the same draws as for amateurs, also represent unfinished business. They are half-cracked, partially deciphered. Why aren't the other two ciphers decipherable? Are they really just gibberish? Giving up on something so fascinating, and something that has frustrated incredible cryptanalysts for over a century, isn't an option to a community of professional puzzle-solvers. Since the second cipher was deciphered, there's a sense of hope surrounding the Beale Ciphers, that maybe they are as real as the story would have us believe. And if they are real, how can cryptanalysts give up? Treasure worth $20 million in today's currency is mind-blowing. Not many people are going to turn their backs on that, especially if they think they have the ability to figure it out.