I should preface this by saying that I really don't know anything about the habitability standards of British submarines. But I don't think it makes much sense to ask why the Type XXI didn't have the same habitability standards as the US fleet boats. For one thing, the fleet boats had a well established reputation for excellent habitability, and that didn't happen by accident. During the interwar years, the US Navy wrestled at length with the question of crew comfort. Unlike other navies, the Americans eventually concluded that the weight penalty of extra crew berths, lockers, separate mess areas, and air conditioning were acceptable. And of course, the Americans were biased toward larger submarines, which could offer a level of habitability that small subs like the Type VII couldn't. But size isn't dispositive. The Japanese I-boats had a reputation for horrible crew comfort, and those boats were huge.

I think the Type XXI offered a remarkably high level of crew comfort compared to any other u-boat, or any other submarine I know of besides the fleet boats. It had food refrigeration, air conditioning, cabins for the officers and petty officers, actual restrooms and a shower. And remember, even the Type XXI didn't have as much internal volume as a fleet boat. For one thing, it was about 60 feet shorter. The Type XXI hull was also full of batteries, far more than a fleet boat. And the automated torpedo room left no space for bunks, so it was still necessary to use hot bunking. But if you read accounts of Type VII sailors, they describe the interior of the Type XXI as being like an "ocean liner."