It didn't happen earlier bc of ur internalized homophobia, sexism, and toxic masculinity bro
@mie_ikaz As someone who reads a lot of A/B/O fics, the location of the bite is important. Yeon Woo bit him on the shoulder, and Keith bit the side of his neck. Older A/B/O settings had alphas bite the nape/back of the omega's neck in order to form a pair, while more recent A/B/O may decide that the "pairing bite" takes place on the scent gland, which is usually described as the side of the neck. Some don't consider pairings to have occurred unless there is a bite + knotting situation.
Basically, we won't know which requirements there are for pairing 'til the next chapter or some other future chapter (unless someone has spoilers). I will note in the very first chapter (where Keith is mad someone marked him in some odd foreshadowing scene) that Yeon Woo is shown with a bite mark on the nape of his neck.