Why did Saitama not get S-Class?

Although the hero is beyond even the most powerful member of S-Class, Saitama isn’t recognized for his abilities due to a broken system that privileged his test-taking ability over his raw attributes, which tested impossibly high.

Why was Saitama given C rank?

This all happened because of his poor academic strength which caused him to get a total of 21/50 in the written part of the Hero Examination thus a total of 71/100 – which was the cut off marks for being a hero and thus became a C Class Hero..