1, To keep secret the level of expertise in cryptanalysis so future opponents wouldn't put effort into improving their own codes. Probably pointless because it was inevitable that some details of Bletchley would leak to the USSR, in the same way as secrets of the Manhattan project. And probably equally pointless in that, although every side in WWII broke almost all of the opponents codes - they all believed their own codes were unbreakable so put little effort in counter-crypt-analysis.
2, The UK government gave captured Enigma machines to their allies - claiming that they were unbreakable.
3, Allied leaders wanted the glory of people believing that they had won through their own bravery and military genius. Not because they had been reading all the opposition's orders before they did.
4, Standard government procedure to keep everything secret until compelled otherwise.
5, Finally and possibly the most important: To not give the losers a simple excuse for why they lost. If the message becomes: Our invincible army only lost because of poor crypto then we would win if we fixed this problem and and so should try again.