The principal reason is that the Pacific is much, much, much, much bigger than the Atlantic. American submarines were expected to operate over much greater distances than their German counterparts.

Second, there were doctrinal differences between the pre-war uboat services. American submarines were known as "Fleet Boats," and were intended to support the American surface fleets as scouting vessels as much as they were intended to attack merchant shipping. While this did not work out entirely as intended, American submarines were still involved in several naval battles during the war, such as the USS Nautilus at Midway, and the USS Albacore and USS Cavalla at the Philippine Sea (which managed to torpedo and sink the Japanese carriers Taiho and Shokaku, respectively). Operating in conjunction with surface battle groups requires submarines to be faster and larger.

The Germans, on the other hand, did not have a sizable surface force, and were disillusioned with using submarines in direct support of naval operations after bad experiences in World War 1. While German submarines did participate in naval operations such as the Norway landings, that was not one of their primary goals. Instead, they were intended to be merchant and convoy attackers first and foremost, and only needed to operate (at first) out into the mid-Atlantic. Secondly, it was in Germany's favor to make the submarines as small and easy to build as possible, given their limited prewar military industry and their need to rapidly build as many submarines as possible during rearmament.

The naval treaties of the 1920s and 30s not only limited the number and size of new ships, but it also put a moratorium on the construction of new bases in the Pacific. Consequently, the US Navy had to develop ships with better range than most other navies.

Sources: Wiki and "Agents of Innovation: The General Board and the Design of the Fleet that Defeated the Japanese Navy" by John T. Kuehn