(phoro from Asashi Sushi)
Maguro/Tuna is undeniably the most popular sushi item in the world, while many say that Toro/Tuna Belly is the King of sushi.
The highly prized toro also comes with high price - $50/lbs. to $100/lbs. at a fish market, which makes one piece of sushi anywhere from $10 - $30 easily at a reputable sushi bar.
Despite it's popularity, tuna was not widely consumed in Japan until Edo period (1603-).
Two main reasons being:
Tuna used to be called "Shibi" in Japanese. It has the same pronunciation as "day of dead" and was considered unlucky to eat.
Japanese preferred Tai/Read snapper over tuna because the word "tai" rhymed with "celebration."
It is also said that it's unpopularity was due to the size of tuna - it can be as long as 15 feet and weigh over 1,000 pounds. Because of lack of refrigeration technology, it was difficult to keep its freshness.
Around 1830, Edo had a large catch of Tuna for many years and had abundant supply of tuna, which lowered its price down. Since it became so cheap, some of the sushi restaurants decided to try using tuna, which caught its popularity.
The way they served tuna is called "zuke" - a soy marinated tuna nigiri. By marinating in soy sauce, tuna lasted longer without refrigeration. It also brought different flavor to tuna. Until then, tuna was either grilled or cooked and was considered not tasty.
Toro, tuna belly started to get popular around 1930. It is unclear exact reason why it became so popular. One theory says it is due to the changing diet of modern Japanese.