1. Past simple refers to a period of time which finished at a fixed point. The speaker's state of ignorance about Agnes's illness finished at the moment when he or she found out that Agnes was ill. That is why the past simple is used - the period of 'not knowing' is over. It is a closed time period. 2. The present perfect is used because the implied time period is 'up to now'. The speaker is looking at the other person at the moment of speaking, which makes it an open time period.

It's a good question. If you rewrote the sentence to add in the implied period, it would be something like: "I didn't know that fact [before you told it to me e.g. 10 seconds ago]" The time of this "not knowing" is a finished period in the past.