Technology evolves. What was top secret becomes less important as time goes on, because newer, better technology replaces it.

For example, in 1955 when the film "The Dam Busters" was filmed and released (see http://www.imdb.com/title/tt0046889/?ref_=fn_al_tt_1) some details of the bouncing bomb were still classified, so the film could not show these details. However, today the technology is considered archaic, there is no viable use for it in todays world.

As far as codebreaking is concerned the Allies success and the whole story of Bletchley Park was kept secret for a long time. Even during the war only a selection few actually knew where the information was coming from and how it was received. The Allies actually chose not to use some of the information they decrypted for fear that it would indicate to the Germans that the Enigma code had been broken. After the war, for fear of leaks about the work that happened at Bletchley (and elsewhere) it was ordered that all Bombe machnes (used to help in the deciphering of messages) should be destroyed.

The people who worked at Bletchley Park were quite remarkable, sworn to secrecy they got on with their lives and never told anyone what they had done during the war. Marriages happened, with both husband and wife having worked at Bletchley Park and they lived their whole lives without telling each other what they had done. It wasn't until the late 1970's that information and personal accounts started to appear.

Bletchley Park itself was used for other means after the war, its wartime use highly classified - in some cases what we would now consider important buildings were destroyed or converted - in one case into a canteen, because the people in charge were not told of their significance.

The other thing to keep in mind is that wartime inventions weren't necessarily new - Radar was used by both sides (Germany had a system called Freya), as far back as 1904 German engineers had applied for and received patents based on radar. What was kept secret was how they were implemented and used.

And as for the other secrets, codebreaking in particular, it seems like the militaries would have wanted to keep those secret in case they needed to use them again. What happened?

Modern cryptosystems work on a completely different method from the ones used in WWII (at least German Enigma). Enigma worked on a rotating substitution cipher, called a rotor machine. Substitution ciphers work by turning letters into another letter, a famous example is the Caesar cipher, where D=A, E=B, F=C etc.. A rotating cipher changes what letters are substituted, press a key on an Engma machine and may A=C, the next A pressed may be E etc.. The problem is that if someone captures said machine, they could encrypt and decrypt your messages rendering them pointless.

Modern crypto-systems are designed that even if someone is observing all parties 100% of the time, they could not be broken. A famous example of this is RSA which is a public key crypto system. In this type of system everyone can encrypt their message using a public key, but the private key is kept secret by one person. So if Alice wanted to send a message to Bob, Alice would use Bob's public key to encrypt her message. Bob would then use his private key (which is kept from everyone) to read the message. If Bob wants to send a message back he would encrypt his message using Alice's public key who would decrypt it using her private key. Even if Eve, a hypothetical eavesdropper (heh) is watching the conversation without those private keys (which are never told to anyone) the information is meaningless to her. I could go into the mathematics as to how we encrypt messages this way (if you want).. but this is ask historians, not ask mathematicians.

The breaking of Enigma was first revealed in F. W. Winterbotham's book "The Ultra Secret", published in 1974. At this point, US and British military encryption had switched from the various electromechanical systems similar to Enigma and were ready to accept declassification. One of the early fully electronic replacements was the NSA TSEC/KW-26, introduced in 1955. Systems of that class were mostly introduced throughout the late 1950s and into the 1960s, making declassification possible.

The exact details of the breaking of the German 10-12 rotor Lorenz cipher, codename "Tunny", was not declassified until 2002 - it was not available to Jack Copeland in 2000 when he wrote "Colossus: The secrets of Bletchley Park's code-breaking computers".

One of the things people find unexpected is that even before World War II had ended, in June 1945, President Truman issued Executive Order 60663, which basically said that any scientific information that was secret during the war would have to quickly become non-secret in the postwar. The basic idea was that they would try to reduce scientific secrecy to a minimum once the immediate expediency of war had ended. It is a very odd Executive Order, in a way, representing a pre-Cold War view of scientific secrecy. It did clear the way for a lot of mass declassification. Much relating to radar, for example, became declassified very shortly after the war (e.g. basic principles, some applications, some breakthroughs, though the specifics of what was currently being used by the military was not). Ditto work on camouflage, and lots of other wartime work. Even something without obvious civilian benefits, like the development of incendiary bombs, was published on, because it wasn't felt it could really effectively be kept a secret, the need to keep it a secret didn't seem that high, and the secrecy could be expensive and prohibitive to further development.

But there were a few areas that fell into different categories. Nuclear weapons were one of them — they were made explicitly exempt from this order, and the story of the release of information about them is a much more complex one (which I am writing an entire book on so I will not try to summarize it here, other than to say that it was neither treated as an absolutely permanent secret nor something that could be easily released). Cryptography was the other — this was treated with even more secrecy than the bomb in many ways, because it lacked a strong public presence (the bomb, after all, had been used) and lacked strong advocates for its declassification (atomic energy was seen as probably having strong civilian benefits from early on, but cryptography, especially code-breaking, was not). As a result it was kept uncommonly secret for a relatively long time — the fact that the US had broken Soviet wartime codes (the Venona project) was not revealed until the 1990s, for example, well after its usefulness as a military or even diplomatic secret had expired.