In my opinion, I think the main reason people are still trying to break the Beale Cipher is based on the 20 million dollars worth in gold that is supposedly buried. Money can make people do crazy things and people often won't think about the thousands of other people that have tried and failed and the chance that there is no treasure to be found! Along with the fact that so much money is at stake, the second cipher was cracked using the Declaration of Independence so people probably feel like there is hope. If none of the three ciphers were cracked then less people would try probably. Some other people probably want to crack the code out of pure pride. Since so many people have tried and didn't succeed, then if the cryptography world would give them a lot of respect.
Image "Fishpool gold coins" by Lawrence OP, Flickr (CC)