Okay so I read the novels and here’s why.
The first reason is that Tae was extremely exhausted, like dead-dead exhausted. He could barely even cry properly and his body was tormented all day. The novel described his exhaustion in depth, how he used up all his might to say what Ilay demanded, and how he felt like a corpse at that moment, with barely any strength left to speak. U see how he struggled saying it the first time? When it was barely audible, Ilay frowned. Tae was terrified that Ilay might continue so he shouted.
BUT the main reason he didn’t say anything was because of the way he felt about Ilay and the situation.
You see how Tae was debating and questioning whether he even liked Ilay and how he thought he was schizophrenic? With the slight hope that he placed in Ilay shattered after this incident, he definitely would want to reconsider saying anything at all. Because to Taeui, what's the point of saying anything to a psycho who did that to him? The guy who even considered killing him even for a moment? Even if he did tell Ilay, there’s no point because it doesn’t undo what Ilay had done to Tae. He meant to tell Ilay before things escalated that far, now that it has escalated beyond far, there’s no point, really.
If you want a SPOILER tho, read the next sentences.
Tae will end up telling him but only after Ilay said certain things that helped make up Tae’s mind to tell him. I’d be happy to spoil more but reading the novel is wayyy better to understand them and the plot. The sequels are especially good.

Taeui wtf why didn’t you tell him right there and then that you actually didn’t want to leave, that you were actually running back to him?!?!?!?! You were literally crying bitterly in the last chapters because Ilay is not giving you a chance to explain, wtfff bro, I almost shed tears for you.