Submarines were very important in the American victory in the Pacific during World War II, but hardly used at all in the Atlantic Theater. Why was...

Submarines were neither practical nor necessary for the
Allied side in the Atlantic Theater.  This was mostly because the German Navy was
heavily outgunned by the British Royal Navy and the Americans, and received almost no
supplies from cross-Atlantic shipping, so there were relatively few targets for Allied
submarines to go after.

Britain depended on heavy aid
across the Atlantic, however, so German U-Boats attacked merchant ships relentlessly,
and the Allies concentrated on building the cargo transports and the destroyer escorts
needed to protect them as opposed to submarines.

In the
Pacific, the entire war was fought over islands, all of which had to be supplied by sea,
so submarines use by both the Japanese and the Americans was a much more natural and
necessary strategy.  So it made much more sense to concentrate all of America's
submarine forces in the Pacific.