The only records we have of cryptography used in the past come from people with the resources and technical skills to encrypt and decrypt messages, whether they were World War II spies, Arab scholars, or Greek military leaders. Although not all of the encryption methods mentioned by Singh in Chapter 1 required exceptional resources (the Spartan scytale method used only a staff and parchment), they all required an understanding of the concept of encryption, which was a largely unused technique prior to the development of each cultures' breakthrough cryptography methods. Additionally, it's a reasonable assumption that cryptographers would have wanted to keep their methods secret from the general public, as knowledge of the code would have weakened the encryption. Therefore, I believe that the reason so few records of cryptography exist outside of well-resourced people is because they closely guarded the secrets to their specific codes after development, which, once revealed, often turned out to be simple and did not require exceptional resources.
However, this only applies to encryption and the building of ciphers. The techniques the Arabs developed for the decryption of substitution ciphers required extensive knowledge on linguistics and math, as frequency analysis only works if the cryptanalyst is familiar with the mechanics of a language.
Over time, techniques for encryption and decryption have been constantly improved in an arms race to create more secure codes and ways to break them. Nowadays, the most secure encryptions are created using supercomputers and unique encryption keys, which arguably requires more exceptional resources than simply deciding on a certain substitution cipher. However, the most significant difference between cryptography now and then is that very secure encryption is available to the general public, while people in the past who weren't involved in the encryption and decryption process had very limited access to effective cryptography. Although only the developers of specific encryptions know the specific mechanics, they are made available for anyone to use.