Did you know sushi first appear in South East Asia and not Japan in the 8th century? During this time people wanted to preserve their sushi longer for travelling and they did this by using fermented rice. In a way, it wasn't for the flavor but the preservation.
It wasn't until the ninth century that Japan receive this food trend from China via the spread of Buddhism. These Buddhist abstain from eating meat thus increased the popularity of Sushi and evolve further by the Japanese.