Mary Queen of Scots fully believed that her cipher was unbreakable, so she laid bare her plan to take control of Scotland. Thus when her cypher was encrypted, there laid a written confession on the table, ready to take her to the gallows. This historical example led to the development of an environment of secrecy and mistrust, where cryptanalysts held the power over cryptographers. Even if one made a seemingly "unbreakable" code, they did not know if another expert codebreaker was waiting to crack it. This never-ending cat-and-mouse game of codes has continued through the centuries, always adapting and evolving. The knowledge that one's code could be broken fostered more caution on behalf of the cryptographer, wherein they sent codes that were more cryptic in nature even in plaintext, knowing that an expert codebreaker might crack their code.
This strategy was a direct consequence of the knowledge that someone more experienced may crack your code - after all, if that was the case, why not make your plaintext message more difficult to understand as well? This would add an additional layer of security, and ensure more protection. This shift was a significant one in cryptography history, and represented a transition to a more secretive/hard-to-decipher language where nothing was taken for granted.