There are questions as to why Saitama one punch man is not recognized as an S-rank hero.
In fact, Saitama’s achievements are still recognized. That’s why Saitama was able to go from C class to B class. In the webcomic version, Saitama one punch man ONE even became an A rank hero.
One-Punch Man Season 2 Episode 2 illustrated why Saitama wasn’t recognized as an S-rank hero.
Saitama casually defeats every monster he meets on the street… but Saitama never reports his achievements to the Hero Association.
Genos stated that even though Saitama did not report his actions to the Hero Association, Saitama continued to rise so B rank was ranked 17th at the top of One-Punch Man Season 2. That meant there was an association representative who secretly observed Saitama’s actions. Very few people know how strong Saitama is. Even characters like Genos and Sonic seem to feel they can keep up with Saitama’s powers.
So, in the superintendent’s eyes, if Saitama had a C class hero (and then B) who could handle a monster by accident, that monster had to be too weak. Though the monster can be Devil or Dragon level. On the other hand, Genos can often be destroyed in battle. But these fights left the monsters Genos faced to truly feel its class, until the power of Genos was recognized.