That's a good question! It all comes back to the fundamental difference between the submarines of WW2 and of the Cold War. The diesel electrics used in WW2 would primarily operate as surface ships, submerging only to attack or hide from danger. With the advent of nuclear power, however, it became apparent that submarines could be designed that would operate submerged almost constantly. And with this idea, people began to design submarines that sacrificed surface performance for submerged performance.
The actual shift begins in 1944, the Germans had pioneered a teardrop hull shape earlier with a prototype midget submarine. The prototype was lost in an accident and the concept abandoned.
It was revisited in 1949, when the United States launched a series of hydrodynamic studies to determine the best shape for a submarine's hull. A program within the US Bureau of Ships to design the best shape for a hull that would operate submerged for most of its operational lifetime. The David Taylor Model Basin tested these designs and came out with the best two. One with a single screw, and one with two, both with teardrop hulls.
Construction then began on an experimental submarine to test the new teardrop hull shape, the USS Albacore, AGSS-569. The Albacore's testing began in 1954 and, deedless to say, the tests were very successful. The teardrop hull shape is even now often known as an "Albacore Hull". The first production nuclear submarines to use the new hull shape were the Skipjack class, starting the USS Skipjack in 1956, and it's been pretty much the standard hull shape for submarines around the world ever since.
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