I think that the "bad-guy-turned-good-guy" theme is incredibly popular, and I think that it was done really well in this story. I also think that people appreciated Dong-Gyun actually ending his toxic relationship with Alex and not getting back into it until he was sure of Alex's change. Ukes a lot of times tend to unfortunately be portrayed as weak and spineless, and Dong-Gyun wasn't (to me anyways). PLUS a broadcast jockey story base isn't something I've ever seen before, so that was new and different. That's all the explanation I've got! ╮( ̄▽ ̄)╭
I think the reason why it's so popular is because of a number of things. Without saying any spoilers, here's a few reasons I can think of from superficial to more specific ones:
1. The art is gorgeous.
2. The sex scenes are hot.
3. Set in real conservative Korea and both characters are gay making it difficult for them which makes the story very realistic.
4. Broadcast Jockeys (BJs) are huge in Korea and the idea that one popular BJ gets with a fan is a very attractive one.
5. The 2 main characters actually have personalities.
6. Their past shaped who they are when they meet and it actually mattered in the way they interacted and how their relationship with each other developed.
7. Character growth for both characters.

What makes it stand out, and why so popular?