Bhagavad Gita 4.2

4.2 This, handed down thus in regular succession, the royal sages knew. This Yoga, by long lapse of time, has been lost here, O Parantapa (burner of the foes).

This supreme science was thus received through the chain of disciplic succession, and the saintly kings understood it in that way. But in course of time the succession was broken, and therefore the science as it is appears to be lost.