[magic452]

The Gato was such a successful design that there wasn't much you could or needed to do to it.
It fit the Pacific Theater so well as to range, armament, speed, etc. there was little need to change except quicker and deeper dive capability.
The Balao and Trench were little more than slight improvements rather than new designs. Could have been called Gato B and C models. If it ain't broke don't fix it.

U boats had to adapt to different operating theaters and conditions.

Magic