Because it was historically inaccurate and made in the overdramatized Hollywood style.

If you know anything about U-Boats, you know that the British were the first to recover Enigma machines and coding documents from U-110 and later coding documents from U-559 in the early half of the war.

It was not until 1944 that the USS Guadalcanal and USS Pillsbury managed to capture U-505.

the movie U-571 was also inaccurate as the Actual U-571 was not captured and sunk in an aerial depth charge attack in 1943 or something.

The Kriegsmarine destroyer "anschluss" seen in the movie looks like it was made from a fishing ship or something. It is definitely not a World War Two era destroyer, especially one of Kriegsmarine origin as very few were made, and any left seem to have been sunk in atomic bomb tests during the cold war.

the depth charge attacks as well were overdone. The proximity of the charges to the hull would have almost certainly sunk the boat at that distance. You hear about U-boats being sunk by three or four dropped from a plane, not 50 poured directly on it from a destroyer directly above.

Its fun to watch, and the special effects are great, but as a U-boat movie, i give it NO respect.