The Great Cipher of Louis XIV was truly a remarkable cipher, and its longevity only attests to its success. Antoine and Bonaventure Rossignol were the masterminds behind its brilliance, and with their death brought the end of its effective use. During the time of its use, the Rossignols lived adjacent to Louis XIV, since they were the only ones who could effectively use it. The fact that it took two centuries to crack is beyond remarkable, and it deserves praise.
The key to its success and difficulty was that the plain text was not explicitly the 26 letters of the alphabet. Most monoalphabetic and polyalphabetic substitutions start with the initial 26 letters (a, b, c, ... z) as the original plain text and substitute each letter with another letter (or in the case of polyalphabetic ciphers) or maybe a couple letters. But the fact is that the plain text and the cipher text will be limited to the 26 letters of the alphabet, no matter the method of cipher used. This cipher was unique compared to others because the original plain text consisted of all of the individual sounds or syllable used, hence there were many, many more original plain text "letters" compared to an ordinary substitution ciphers. Therefore numbers had to be used rather than letters because there was no alphabet large enough to contain all of the syllables in the language.
Additionally, the Rossignols added traps within the cipher, such as numbers that adjusted adjacent numbers (like removing them entirely). Numbers also often translated to single letters rather than syllables which threw off people who attempted to decipher it. These key differences made the code inefficient since there were so many different "letters" that a person needed to keep track of, but at the same time made it virtually uncrackable to even the most scholarly people. If it were not for the sheer size of the pool of the total numbers used, people would have undoubtedly continued to use it.