Bhagavad Gita, Chapter 4, Verse 2

Thus handed down in regular succession, the royal sages knew it. This Yoga, by long lapse of time, declined in this world, O burner of foes.

Even something true and carefully passed down can still fade if no one keeps carrying it forward.

This isn't really about ancient history, it's a real pattern: knowledge held by fewer and fewer people, generation after generation, eventually thins out and disappears. That's exactly the gap Krishna is about to fill back in.