Once again, I'm here talking about Weak Hero Class...
I've been re-watching Season 1, and it's becoming easier to see from the beginning why Sieun and Suho would inevitably become close, while Beomseok was never going to share the same relationship with either of them.
If you really watch how Suho and Sieun interact in the beginning, they become close because of how brutally honest they are. This is especially one of Sieun's most endearing qualities, and is something that draws most of the characters all the way to Season 2 to him (even Seong Je and Baek Jin are fascinated by Sieun because of how bold he is).
In fact, he is so honest that he is actually unimpressed with Suho and his charisma at first (e.g. his reaction after the baseball team fight).
However, Sieun also quickly recognizes that Suho has a hard life. After watching him work his delivery job, he studies in the dark to give him time to rest. When they eat together, Sieun asks Suho why he sleeps at school, and learns that he is committed to finishing high school because of his grandmother.
Later, while helping Suho at his restaurant, Sieun asks him if he has multiple jobs, and asks him, "Isn't it hard?" This is one of the few times we really see Suho in a vulnerable moment, as it seems to catch him off guard. Was there anyone else who had ever asked him that before? He turns away and says, "It is what it is," then opens up that he actually has yet another job as a mover on weekends.
At that moment, it clicked for Suho that Sieun wanted to know him. That he was curious about his life and his thoughts, and understood that he was struggling to make ends meet. Suho is also comfortable enough to jokingly ask if he respects him more now; he knows that Sieun is not looking down on him or feeling sorry for him - he is simply being sincere in his desire to get to know him.
Meanwhile, Beomseok becomes interested in Suho after hearing how popular he is, and after hearing rumors of him being an MMA fighter. Even though we see him desperately wanting to be Suho's friend, we don't actually see Beomseok asking him questions about his life, and truly wanting to know him in the way that Sieun does. Later on, we actually see Beomseok use his economic struggles against him after confronting his old school bullies ("Do you want me to pay your wages?"). Due to his own insecurities, he is unable to comprehend that someone as confident and charismatic as Suho could be struggling through hardship too. This clearly hits a nerve with Suho and is a major turning point in Suho and Beomseok's friendship.
May add more to this later, but these are my thoughts on one of many reasons why this trio faced the fate that they did.
