Using the past simple tense is correct because it implies that you didn't know something. But now, you know about that because someone said about that to you. Using the present perfect tense is incorrect because it implies that you didn't know something, and still don't know about that, which is incorrect because someone said about that to you, so you know about that. So, you should not use the present perfect tense: "I haven't known .....", but the past simple tense: "I didn't know .....".

The first one is said in English. You were in a state of not knowing, but now you are not. So the simple past conjugation is used.