The Buddha is said to have rediscovered the ancient path. Why was it lost to begin with? My understanding is that the Buddha is said to have &quot;rediscovered an ancient path&quot; which, presumably, formed the core of his teachings.
Source: Gil Fronsdal’s Introduction to the Dhammapada.
My question is: Why was it lost to begin with?
And why is it that the teachings of the Buddha are so voluminous and contain so much redundancy?
UPDATE: I am absolutely convinced that the Buddha's 8-fold path leading to the end of suffering does so by training the mind to respond to the pain of unskillful moves caused by unskillful predictions by (1) restraining the hindrances and (2) investigating with the intent of processing the feedback for insights to improve the predictive model of the sensory-motor brain so that more skillful predictions leading to more skillful moves will be made.
Why does this lead to the end of suffering? Because although pain is inevitable, suffering is a choice. Specifically, suffering is the choice to cling to wrong views because doing so leads to the descent into the hell of uncertainty. As the world descends into chaos, those who cling to views will descend into chaos along with it because they are refusing to &quot;see things as they actually are.&quot;
In other words, clinging to &quot;wrong views&quot;, views which do not correspond to the evidence of sensory experience, is unskillful.
Accordingly, please offer only answers that do not violate the scientific method of inquiry. For example, claiming that a human being lived for 80,000 years without offering evidence to support this extraordinary claim is not desired.
OP: My question is: Why was it lost to begin with?

The reason for this is elaborated in SN 20.7.
SN 20.7 shows HOW the teachings will be forgotten - when monks (and I guess also lay Buddhists) do not want to listen to the teachings of the Buddha or pay attention or apply their minds to it. Instead, they will listen to the teachings of unenlightened teachers or poets which are phrased in fancy ways.
Even today, we can find a number of new age gurus in the last 50 years, who have a lot of followers, writing bestselling books or giving speeches using fancy jargon or technobabble such as &quot;quantum healing&quot; or &quot;inner engineering&quot;, and appearing on TV shows or YouTube.
Also, it's not unusual that people start forgetting or misunderstanding the original teachings after a long time. For e.g. in this answer, you can see that the Buddha originally said that he is not 100% omniscient in MN 90 and MN 71, and only said that he had the three knowledges, as well as he had the capacity to know and understand all things, but did not actually know and understand all things. Then Milindapanha, written many centuries later, exaggerated the Buddha's omniscience and made the same kind of excuses that the Buddha himself criticized in MN 76.

OP: And why is it that the teachings of the Buddha are so voluminous and contain so much redundancy?

The Buddha taught for something like 45 years from his enlightenment to passing away. That's why it's so voluminous.
Why does it have so much redundancy?
Firstly, he taught for around 45 years, so it's no wonder that teachings get repeated.
Secondly, I believe that the monks edited and reorganized the Pali suttas over centuries to make them have mnemonic formulae and they also modified similar stanzas to become exactly identical - which is why you can find some standard formulae repeated in multiple suttas. This made it easier to memorize, recite and transmit. You have to remember that it was an oral tradition before it was written down. More details can be found in this answer.
P.S. This answer of mine has paragraphs copied from other answers of mine, and combined together with new sentences, and adds quotes of your question. So, even my own not-so-voluminous answers have redundancy / repetition. So, it's no wonder that the suttas can have redundancy.