In the old days sushi was totally different. It was made by preserving fish in fermented rice. The rice was thrown out!
They eventually added vinegar to the process to improve it. The vinegar worked so well to preserve the fish — they did not have to ferment the rice anymore. That meant they could eat it!
That's also the history behind sushi rice — it explains why sushi rice (sumeshi 酢飯) is prepared with vinegar.