Why the Type XXI U-boat was evolutionary rather than revolutionary

Was the Type XXI U-boat a revolution rather than an evolution?

No; it was a powerful integration of ongoing developments, which is how many apparently revolutionary warships emerge.

Type XXI combined developments that had already appeared in other submarines. HMS Warrior and Dreadnought provide comparable cases in which the new contribution lay in assembling existing ideas into one ship. Germany had a particular reason to bring those strands together, while other navies were pursuing different requirements. Once the combination appeared, those navies could see its value and adopt similar thinking.