As a Creator, Snape can reverse Sectumsempra

Why has so much magical knowledge been lost since ancient times?

I'm not an expert on the history of magic, but I'll do my best .

At the beginning, roughly before AD 900-1000 , The wizarding community lived peacefully with Muggles.

Especially before the widespread spread of Christianity, magic was an important part of some religions, to the point that multiple cultures included a god / goddess of magic: Hecate in Greek mythology, Isis for the Egyptians, and so on. It is likely that some priests were dedicated to the specific cult of such deities, and when this is the case, magicians were not only accepted, but also considered very important people.

This means that wizards and witches at the time were absolutely free to experiment spells, potions, and the like, and in some cases even encouraged by society (e.g., the strengthening potion may have been used at the Sed festival when the Egyptians performed a series of rituals to renew the pharaoh's strength).

Between 900 and 1000 AD Do muggles persecute, at least in Europe, wizards, or at least no longer tolerate them. This is suggested (in the universe) by the fact that Hogwarts was founded in a remote region.

far from arousing muggle eyes too curiously

The founders build Hogwarts and the new generations are taught magic. Given the school's remote location and the contemporary founding of Hogsmeade, it is likely that wizards lived in a close community a little next to Hogwarts. The proximity of many wizards, and possibly the need to update the magical knowledge imparted to students, contributed to intensive magical research and development.

Outside the Universe: Around 1144 an Arabic book on alchemy was translated into Latin by an English Arabist. This marks the beginning of a new wave of magical research related to alchemy and also to potions from well-known wizards like Paracelsus.

The 12th century is a very important period in magical history, also because of the Peverell brothers (see Who Came First; The Four Founders or the Peverell Brothers?).

If we can now compare Muggle and wizarding history, it is during this period that the first (European) books on magic are published, probably by combining old manuscripts with the authors' personal experience.

This is a crucial time: The magical knowledge that is neither written in the first books nor taught by the founders goes within a few centuries lost due to the limitations of oral tradition, etc.

The 15th century marks the beginning of an intense phase of the persecution of witches and wizards. Almost two centuries later, wizards think that something must be done to protect their world: they are beginning to isolate themselves from muggles and build a new society.

According to Dumbledore's notes too The Tales of Beedle the Bard the Department of Mysteries was founded in 1672 (if not earlier) and is older than the Ministry of Magic. The Unspeakables - if that's their original name - do research on a wide variety of topics, including time, love, and death.

The International Statute of Secrecy was signed in 1689 and 1692 officially introduced. We don't know if there was a "law enforcement agency" to enforce the statute, but this decree marks the final separation of the wizarding community from Muggles.

In 1707 the Ministry of Magic is founded. Another crucial point: before the Ministry of Magic was founded, wizards lived - more or less secretly - with Muggles and therefore did not need adequate employment in the wizarding world. This means that they are in their spare time with no legal or bureaucratic restrictions Experiment with spells, potions, etc.

Since its inception, the Ministry of Magic has not only become the most important institution in the wizarding community from a political point of view.

It is estimated (is there any estimate of how many people work for the Ministry of Magic?) That at the time of the events in the books, nearly 1000 out of 3000 wizards / witches were working for the Ministry. Most of the remaining adults have Ministry and / or Hogwarts-related jobs, such as healers in St. Mungo's or shopkeepers in Diagon Alley, and children study at Hogwarts.

It seems reasonable to assume that this percentage of the Ministry's staff compared to adult wizards has remained more or less constant over the centuries. Since the Ministry of Magic was established, many adult wizards have worked for the Ministry or Hogwarts, and as a result have had much less time to develop new spells , and so on.

But there is a much more important consideration: legal regulations .

The Ministry of Magic regulates every aspect of the wizarding life, including things like kettle thickness. In particular, the ministry not only bans pre-existing spells like the unforgivable curses, but also restricts the development of new forms of magic.

Examples:

  1. Prohibition of experimental breeding: restricts the development of subjects related to the care of magical creatures
  2. Registration of Prohibited Charming Objects: Restricts developments related to charms

The only "Center for Wizarding Research" is the Department of Mysteries, which conducts classified research on fundamental subjects such as death, love, time, and space.

It is unclear whether they just did research practice or even develop new forms of magic, since the veil probably wasn't created by the Ineffable (see Who created the veil in the death chamber and why?), but the Department of Mysteries may have invented the spell that automatically contained the prophecies in the Hall records.

The secrecy behind which the Department of Mysteries works restricts the knowledge of fundamental subjects by lay wizards who must obey the Ministry's regulations and are therefore limited in the types of magic they can develop.

In addition, limited the fact, that some kinds of magic cannot be developed, the secrecy of the few wizards who deliberately break the rules of the Ministry of Magic and I am not only talking about Voldemort but Hagrid (who, if so) was not for Dumbledore, would be likely brought to Azkaban).

The assistant community can be divided into the following categories:

  • People with no time and / or advanced knowledge to develop new magic (the Weasleys, the Malfoys, ...)
  • People with advanced knowledge of magic who develop new magic in accordance with Ministry regulations (at least initially Dumbledore and possibly Snape) and be able to share their knowledge publicly
  • People with advanced knowledge of magic who develop new magic and don't care about the Ministry (evil wizards like Voldemort, but also Hagrid) and therefore cannot pass on their knowledge publicly

Prior to the establishment of the Ministry of Magic, wizards had the time and advanced knowledge (which had recently disappeared unless it was written in books or taught by the founders) to develop new types of magic without any legal restriction.

Since 1707, most wizards have had less time and advanced knowledge to conduct research on magic, and the few wizards who spend their time practicing magic either abide by the Ministry of Magic's rules or risk being taken to Azkaban. Those who illegally research magic will not be able to publicly describe their discoveries / inventions or they will be prosecuted.

As a result, ancient magical knowledge has been lost, and magical research and development is very limited.

A. Darwin

Exactly! Couldn't have explained it more precisely.