Mahayana revealed from Naga world to 1st century CE+ - Why?

Please see the article "Buddhism in India before the 13th-Century Invasions" by Dr. Alexander Berzin, and also the book "Sects and Sectarianism: The Origins of Buddhist Schools" by Bhikkhu Sujato. Also, the book "Buddhist Sects and Sectarianism" by Bibhuti Baruah.

The Mahayana perfection of wisdom (prajnaparamita) sutras did not suddenly appear out of nowhere 400 to 900 years after the Buddha's passing away.

About a century after the Buddha's passing away, presectarian Buddhism split into Mahasanghika and Sthaviravada, and over the next few centuries split further into many more so-called early Buddhist schools. Please see this hypothetical combined list of early Buddhist schools on wikipedia.

Berzin's article above partially traces the origins of some concepts in Mahayana Buddhism to these early Buddhist schools.

It appears that while Theravada Buddhism (c. 240 BCE) originated from Vibhajjavada which originated from Sthaviravada, Mahayana Buddhism derived its concepts from various early Buddhist schools including Sarvastivada, Dharmaguptaka, Lokottaravada, Bahushrutiya, Chaitika and so on.

Interestingly, Sarvastivada and Dharmaguptaka descended from Sthaviravada, while Lokottaravada, Bahushrutiya, Chaitika descended from Mahasanghika. Nevertheless, they all had influence on Mahayana Buddhism. However, the majority of the influence came from Mahasanghika and its descendant schools according to most western academic scholars.

From Berzin's article, you can trace back present day Mahayana concepts to these schools for e.g. the fallibility of arhats and transcendental nature of the Buddha from Mahasanghika, Tibetan mantra-chanting from Dharmaguptaka, the three bodies of the Buddha from Lokottoravada and Bahushrutiya, the Buddha being enlightened and transcendental even before appearing on Earth from Chaitika, and so on.

This post by Javier Fernandez-Viña on SuttaCentral traces the tathagatagarbha (Buddha nature) concept in Mahayana Buddhism to Mahasanghika, and provides the scholarly sources. It refers to the book "Buddhism in the Krishna River Valley of Andhra" and Alex Wayman's 1978 paper "The Mahāsāṃghika and the Tathāgatagarbha".

Another interesting article is "The Theory of Two Truths in India" from the Stanford Encyclopedia of Philosophy. Here, we can see that Vasubandhu in 4th century CE who founded the Yogacara school, clearly spent time studying and commenting on the works of the Sarvastivada (237 BCE) and Sautrantika (50 BCE) schools that came many centuries before him.

Bhikkhu Sujato explained in this post on SuttaCentral:

It is true that certain aspects of Mahasanghika ideas were influential in the much later formation of the Mahayana. But so too, were aspects of the Sarvastivada, and other schools as well. Mahayana emerged as a reform or reaction against multiple aspects of the early schools as they existed around 500 years after the Buddha. It didn’t stem from any one school. In terms of Mahayana philosophy, the greats such as Asanga, Vasubandhu, and Nagarjuna are much more closely linked to Sarvastivada than Mahasanghika.

So the Mahayana prajnaparamita sutras did not suddenly appear from the nagas in the sea. Rather, its concepts evolved gradually from the early sectarian era of Buddhism.

Most if not all of the Mahayana doctrines are present in the Pali Canon in their basic form, often as mere ideas implied by brief Buddha's utterings. Since Pali Canon cannot possibly be 100% exhaustive account of everything Buddha taught it is safe to assume there were other teachings or details of teachings not captured by the suttas.

We know for a fact that after Buddha's death there were centuries of speculative analysis and systematization of his teaching. The Samyutta Nikaya was compiled and organized by topic, then the Anguttara Nikaya, then the core concepts in these two were extracted and boiled down to the matrika lists, which were then elaborated upon to create what's known as the Abhidharma.

This happened to be the same historical period when India went from zero literacy to one of the strongest textual traditions in the world, the trade routes between West and East were established and facilitated the exchange of ideas between Greek, Persian, Indian, and Chinese cultures - all great in their own ways, and a big chunk of the Indian territory along the Ganges fell under Greek rule.

This was a period of tremendous development for India in all areas - ancient technology, natural sciences, medicine, mathematics, linguistics, philosophies of all kinds, even traditional Hindu religion developed new theories and teachings.

In addition to that, what at the time of Buddha were temporary camps of otherwise homeless wandering monks, developed into permanent monasteries with dedicated research staff specializing in Buddhist studies.

Imagine this environment spending several centuries reading every sutta thousand times over and analyzing them from all possible angles debating their obvious and hidden meanings. This was the vogue of the times. The best minds of India were drawn to this then highly respectable and intensely satisfying activity.

Naturally, some of the ideas present in suttas in the seed form were stumbled upon by the ancient scholars again and again, in different places at different times. Connecting the dots the later scholars gradually came to a realization there was a definite layer of meanings in the suttas beyond the superficial.

This layer of meanings came to be known under the moniker of prajna-paramita, the transcendental wisdom, or perhaps the transcendence of wisdom. It took northern India about 4-5 hundred years working by trial and error to churn the milk of the suttas into the butter of prajnaparamita, and to recognize that this, indeed and beyond slightest doubt for those who carefully studied the suttas, must be the heart of what Buddha-Dharma was all about.

The Nagas and Dakini realm in Tibetan Buddhism is a well-known and rather opaque metaphor for the abstract realm of latent potentials. In this context it means that the teaching of prajnaparamita was there in the seed form but took time and human assistance to get developed into a detailed doctrine.