Via io9: ‘Nigiri is… a relative newcomer to Japanese cuisine, invented some time during the 19th century. A sushi shop owner named Yohei Hanaya is often credited with created the hand-squeezed nigiri, but he may have just been the most successful early vendor of the dish. But nigiri definitely got its start in Edo, the city which was renamed Tokyo just a few decades later.
While nigiri quickly became the most popular style of sushi in Edo, it did not immediately dominate the sushi landscape as it does today. In his book The Story of Sushi, Trevor Corson credits two events with the rise in popularity of nigiri outside of Tokyo: One is the Great Kanto Earthquake of 1923, which forced many people (including sushi chefs) to leave Tokyo for their hometowns. When the Tokyo sushi chefs opened up sushi restaurants back home, they made Edomae (Edo-style) sushi, with an emphasis on nigiri.
The other event is where the Americans come in…. During the American occupation after World War II, a food rationing program helped the rise of nigiri outside Tokyo…’