You're right that it took a long time for the work to become public knowledge, but it wasn't 50 years. The work on Enigma and the general cryptography and military signals intelligence work at Bletchley Park, under the 'Ultra' umbrella, was declassified in the 1970s. In the first few decades after the war it was kept strictly a secret, partially because of standard protocols, but there was also a very practical, very devious, rationale...

Once WWII had ended the UK sold captured Enigma machines to allied and non-aligned nations on the basis that they could use them for fool proof secret communications. British Intelligence did not want it to be public knowledge that they could very easily intercept and read these messages because it was still a useful intelligence gathering tool for keeping an eye on foreign governments.

The 1970s declassifications only occurred once advances in affordable computing hardware had rendered the Enigma machines clearly obsolete, not just obsolete in the eyes of British intelligence.

British intelligence used their Typex machines, which were based on Enigma but were more complex, only until the 1950s, however they also sold these to allies on the same basis as the Engima machines until the 1970s.

http://en.wikipedia.org/wiki/Typex#Security_and_use

http://en.wikipedia.org/wiki/Ultra

F.W Winterbotham The Ultra secret: the inside story of Operation Ultra, Bletchley Park and Enigma.

It seems that under the UK Public Records Act of 1958 (http://www.legislation.gov.uk/ukpga/Eliz2/6-7/51/section/5/enacted?view=plain), classified records were to be kept as such for 50 years (it's now 20 years: http://www.telegraph.co.uk/news/politics/10213570/New-20-year-rule-means-Government-records-will-be-released-earlier.html).

There seems to be a procedure for declassifying records sooner, though, and I can't say why that was not done.