Gita 04.02 explained
imaṁ rājarṣayo viduḥ
sa kāleneha mahatā
yogo naṣṭaḥ paran-tapa
Word for word:
evam — thus; paramparā — by disciplic succession; prāptam — received; imam — this science; rāja–ṛṣayaḥ — the saintly kings; viduḥ — understood; saḥ — that knowledge; kālena — in the course of time; iha — in this world; mahatā — great; yogaḥ — the science of one’s relationship with the Supreme; naṣṭaḥ — scattered; param–tapa — O Arjuna, subduer of the enemies.
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.
Standard way of receiving this knowledge is being in succession.
Raja and Rishi are totally different. Raja is external ruler but Rishi controls the inner world, when both of them are integrated in one person he is RajRishi, (Saintly King).
Sometimes people when read or watch Mahabharata there are asking so many question about why gambling etc. was done, so we can see here that when Shri Krishna is saying that knowledge is lost that means at Mahabharat time the knowledge was lost and actual culture was deviated. And Mahabharta time was much better than the present time.
Krishna is saying by the power of time, it has degraded, by the passage of time milk becomes curd and curd is spoiled, but if we want butter then automatically it will not happen there has to have positive energy put in doing so, then that curd will become butter and Ghee can be made from it, that is positive transformation.