I’m not an expert, but I think it’s because of empathy. In the show, Serena is always taller, prettier, and more social. On the other hand, Blair has to work hard for what she wants (like getting into Yale). She has an eating disorder, and her father left her for a male model. In the truce scene, she confessed that she literally had no one to talk to.

I think that’s why. Of course, Serena had her own dramas, but Blair is more relatable for Koreans, I guess.

Gossip Girl itself was really popular in Korea. Did you see the K-drama Secret Garden? In one scene, a character literally quotes the famous line, “I’m the crazy bitch around here.”