Why did information about Ultra and Colossus remain secret after World War II?

Information regarding the Ultra project and the Colossus computers remained classified after World War II because the operations were conducted under an injunction of strict secrecy that was not lifted even after the conflict ended.1

Because these machines and the intelligence they provided were vital to the Allied war effort, the British government maintained this secrecy for decades to protect the methods and technologies used to crack German ciphers. Following the war, the existing Colossus computers were disassembled. The public did not begin to learn about the achievements at Bletchley Park until 1974, when Frederick William Winterbotham received permission to publish his memoir, The Ultra Secret. Most detailed information regarding the Colossus machines remained classified until the 1990s.21