The Buddha is said to have rediscovered the ancient path. Why was it lost to begin with?

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My answer: (condensed form)

The Buddha offered his teachings in a voluminous fashion with insurmountable redundancy. He could have offered his teachings in a far more condensed fashion. The Buddha was not stupid. So why did he choose to do this?

Throughout all of history religious teachings which challenged the authority of the ruling class have been co-opted by the ruling class. The Buddha knew this because he himself had been forced to re-discover the original teachings after they had been corrupted. He knew that it would happen again. He chose to revivify the teachings by adding volume and redundancy as a defense mechanism to ensure that they could be rediscovered again by a sufficiently motivated disciple.

Your second question is the easiest to answer:

why the teachings of the Buddha are so voluminous and contain so much redundancy?

Because for centuries they were transmitted orally. Anthropologists have observed that repetition in oral transmission is ubiquitous. Repetition is a helpful tool against oral dilution often compared to the "Telephone Game"

Your first question is harder to answer, since it is somewhat open to interpretation:

the Buddha is said to have "rediscovered an ancient path". Why was it lost to begin with?

The question assumes this "ancient path" was actually lost to the human experience. This is not necessarily the case. Many schools of Buddhism actually teach that the mental state that precedes attainment is something we have all experienced at some point in our day-to-day lives. We just didn't notice or weren't able to place it. Those that have practiced meditation for a long time will often describe the first time they consciously attain this mental state as this being "familiar", i.e. something they have experienced before.

In this light, Buddhism is an upaya, a skillful means or tool to learn to experience this mental state, often symbolized as "a boat to get you to the other side of the river". It is not meant to be carried on the back inland once you reach the other side. This is what they mean in Zen when a teacher says a student's answer "stinks of Zen". The student is trying to "translate" the experience into Zen vocabulary. Since language is by nature dualistic, it is not possible to render any human experience fully into language without aspects getting "lost in translation". The "ancient path" therefore can be thought of as the actual experience of this mental state, which is universal and described in many cultural traditions, but gets "lost" in translating it into the local vernacular.

Why was it lost to begin with?

Because even the Teaching is a part of conditioned phenomena. The only un-conditioned is Nibbana. For everything else, they're all subjected to Anicca

And why the teachings of the Buddha are so voluminous and contain so much redundancy?

The Buddha was a great teacher who had a long lifespan. As a result, he was able to propagate the Dhamma for more than 40 years, hence the voluminous colections of Buddhist text. About the repetition/redundancy, it was for ease of memorization, an absolutely vital component in the preservation and propagation of His words during a time when information could only be transmitted verbally.

I will answer your one question.

Why does this lead to the end of suffering?

When you refrain from sexual activity and train your mind to follow the 8 noble paths, after some period of time your mind will gain the power to immerse itself in the meditation. When your mind goes deep in the meditation, even the state of wakefulness feels like a dream, and at this point pure pleasure and rapture form in your body. This pleasure is so great that nothing in this world would compare, you become very ecstatic, and by giving a continuos effort in this practise, you will permanently be free from suffering and worldly things.

Ah, well...

There is a trace of perennial philosophy here that escapes most people. Perennial philosophy is the idea — common to most mysticisms — that there is one ultimately true (real, transcendent...) understanding of the (human) world that all faiths, religions, and philosophies bend towards. Gautama Buddha taught a path that led there, but he didn't create the goal or the path. He found what was already there, and set out to show us how to see it for ourselves.

I mean, to be 'Buddha' means to be awakened to this ultimate truth, right?

• Was Gautama Buddha the first to awaken to it? Hmph...
• Was Gautama Buddha the last? Hmph...
• Do the answers to those questions make his teachings any more or any less profound? Hmph...

The perennial philosophy is perennial because as soon as it's found it begins to be lost. One cannot transfer an understanding; one can only point the way and help people navigate the path on their own. The thread of the teaching begins to unravel as it is taught, and as it unravels people begin to cling to it: adding commentaries, repeating points they want to emphasize, creating new practices and discourses. It's a bit like a waterfall, where what starts as a pure, clear stream bounces around, tumbling and churning until it dissolves into an inchoate mass of white water and spray; and then someone finds the stream again. Such is life.

For the same reason Buddha's teaching was destined to disappear, because of people losing its deep meaning and replacing it with the superficial "pegs":

SN 20.7

Staying at Savatthi. "Monks, there once was a time when the Dasarahas had a large drum called 'Summoner.' Whenever Summoner was split, the Dasarahas inserted another peg in it, until the time came when Summoner's original wooden body had disappeared and only a conglomeration of pegs remained.

Here the drum still looks the same, it's the deep voice that is lost.

Deep insights like the Dharma are notoriously difficult to convey, therefore difficult to understand, difficult to retain, difficult to pass on. The simplistic and the superficial is what remains, not the subtle and deep.

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 "quantum healing" or "inner engineering", 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.