There were a multitude of practical considerations. For one it took time for decrypts to be placed in the hands of operational commanders who could take action, in many cases making it of less value. Even in instances when decrypts provided timely and accurate information, such as at Monte Cassino in Italy it did not assure victory. The German forces involved were known and their locations and readiness revealed. But Allied strategy and the strength of German positions still held them up for weeks, and when the lines were breached German tactical and operational leadership was still able to conduct a fighting withdrawl, in fact the German command network had broken down so badly that for a few days neither side really knew the actual situation on the ground, SIGINT isnt all that helpful in situations like that.

Then there was the regular changeover of codes and code books, meaning that decrypts would lose value and need to be worked on again. There was also the fact that not all German/Japanese codes were broken, or readable at the same time, and the Axis did have some success breaking Allied codes.