It’s all the daddy’s fault.
Clearly you can see it that he was against his own son dating another guy. Since the 1st season, I figure that he contacted De and gave him an offer that he couldn’t refuse.
When Yi saw him, he was with his family (not direct). She calls the baby, baby, and not De, but with the action that took place and the form they were, Yi misunderstood, and it make sense since De was already back, and he never contact Yi.
But at the end, daddy Yi is the reason why De never contact him. At the end, he will become the head of the organization and this will make Yi look bad. And this is my other theory. De knows about Yi’s family and the opportunity that he might have. That’s why he never contacts him. Daddy Yi basically told him that if he keeps dating his song (Yi), he will personally remove Yi from his life, money, and future opportunities. So, De took the bullet!