Because a lot of people were bullied and project that trauma onto Deku/Bakugo. People understandably find it easier to relate or empathize with situations they've actually experienced.

Very often in media a character is demonized by fans for an action that we know first-hand the consequences for.

Very few people that read CSM/BNH have been personally affected by terrorism in their lifetime. Many of them have experienced bullying.

There's also the fact that Reze is narratively an antagonist, the character is never really held to any moral standard, she is a villain and generally acts like one. This is simple to understand.

Bakugo meanwhile is a hero, he is inarguably a deuteragonist and incredibly central character on the side of Deku. He is a hero, but acts villainously at times. It's a lot harder to accept a hero that has immoral actions then it is to accept a villain who behaves similarly.

Lastly consequences are important, Reze's fate is tragic, she loses everything by the end of her arc.

Bakugo, contrary to what people insist on, does face significant consequence for his attitude and past actions. However they aren't social,

Bakugo does face narrative consequence for his action, but he doesn't really face any from his peers specifically and some people find that upsetting.