As more and more people gained knowledge of the existence of cryptography, the possibility of more secure communications wouldn’t have been all that unappealing a prospect to the common man. In the later 1800’s, education would have been more prevalent so literacy rates would have gone up significantly. A more educated population could better see the benefit of secure communications and records. Information like business secrets could be encoded to give some degree of deterrence to thieves, and messages sent through an easily intercepted medium such as the postal service would be more secure.
Additionally, with the invention and widespread use of the telegraph, the number of communications greatly increased. Of course, due to the nature of the telegraph, unless you had your own relay and message station, all messages had to be sent out through an operator. The idea that all telegrams would be read without it even being intercepted by a third party would be unnerving for many. This doesn’t even account for the possibility that a telegraph operator might be paid to reveal important messages despite their being sworn to secrecy. Even a simple level of encryption would prevent nearly every instance of a private message being intercepted, barring a third-party interception through the telegraph line.
Today, there is also an interest in ciphers among younger people in trying to keep short messages secret. However, for most people, a simple encryption of a message takes time, and deciphering it is very easy with the invention of digital computers (along with the hundreds of web tools that can decipher a message in seconds). I feel the general public today are more interested in the overall security of a communication method and not necessarily the mechanism behind its action. For example, while many people might know an “https” means a secure connection, they might not care about the mechanisms of the public key encryption used to ensure that security. In general, they are more focused on the end result of various security measures, and knowing the overall degree of protection given.