The popularity of sushi in Japan may have been associated with Buddhism.
Buddhism was imported in the 6th century AD, and quickly became one of the major religions. One of the rules of Buddhism was to restrain from eating meat, and fish was not counted as such.
Until 1923, sushi was primarily served as street food.
After the 1923 earthquake that struck Japan, owning a restaurant became more accessible due to a rapid drop in property prices, and such estates gained popularity.