Because Saitama Does Not Want It To Break.

I... I just realised that Saitama's Hero suit got vaporized extremely easily by the first direct attack from Garou after Saitama stopped believing in himself to be a hero. It's a symbolic event.

On the other hand, Genos core is staying intact against nukes and gravity bombs and moon destruction by sheer force of will.

Because Saitama does not want it to break.

But the thing is, his suit has always gotten shredded up just a bit. Holes here and there. His suit is very sturdy, but not impervious.

Because Saitama never fully believed himself to be a hero. Not completely. He was just a hero for a hobby.

He even relates to Garou by thinking it's weird that Garou would call himself a monster, just like he pretends to be a hero in his mind.

But Genos... Genos...

He's always patching up Saitama's hero suit. Saitama asks him to patch up his hero suit nowdays. Genos worries for the state of Saitama's hero suit too. (albeit in a comedic way)

Because Saitama needs Genos to believe in him. That he can be a hero. That he can change himself to become a true hero. Since he lacks that utmost faith in himself.

;_;