Why? Because you've just told them! It's like when saying "Did you know strawberries aren't berries, but bananas are?", I didn't know if you'd already known about that factoid prior to my "question", but at the time I say it onward, I know you know it because I've just told you. Therefore, it'd be reasonable to use the past tense with "did".
I saw this sentence in the book 1Q84: It's not just you that don't know. However, I am having trouble figuring out why (or whether) this is the correct way to write the sentence, as opposed to: ...