I'm sure the real reason is that Riot can't fit all of the mystery into one story, but I find it weird that out of all of the things mentioned in his journal (Darkin, Void, and Yordles), he failed to mention the Iceborn and Frozen Watchers. Either they're not well-known, or they are very well known. Regardless, I've noticed that various factions don't seem to venture into the Freljord that much. Besides the cold, why?
If you're in Shurima or Ionia you probably don't hear much about the Freljord. If you're in Valoran I'd imagine it ranges from cropping up in conversation to HOLY SHIT BARBARIAN ATTACK RUN FOR YOUR LIVES
Might be because of all those raiders AND the cold.
Edit: the Freljord is probably well known as a harsh land filled with raiders, barbaric tribes, and other savages based on how they interact with the rest of the world (stealing and breaking shit like Vikings). So I doubt their ancient history is well known outside of their tribes. Hell its not well known within their tribes because Lissandra has been erasing evidence to make it less likely they'll be defeated when they return.
Ionian Bard actually talks about Frozen Watchers in Bard Teaser.
Do the Freiljordians even know that much about the Frozen Watchers? Only Lissandra is shown to be aware of them, and that's because she was alive when they were around.
