When did Egyptians stop being able to read hieroglyphs? And why?

In 1799, while on campaign in Egypt as part of Napoleon's military expedition to the country, Lieutenant Pierre Francois Bouchard took notice of a stone that his troops had dug up and were incorporating into a fort near the city of Rosetta, known in the modern day as the city of Rashid. Upon closer inspection, it turned out that this stone had three writing systems carved into it. The bottom was Greek, while the other two were Egyptian scripts, Demotic and Hieroglyphics respectively. Eventually, after painstaking effort to translate it, this artifact, known forever after as the Rosetta Stone, played a major effort in the decipherment of Egyptian writing. The first evidence we have for the employment of Hieroglyphics comes from a seal, located in a tomb dating to either the 28th or the 27th century BC, although there is an argument that another site may have an earlier example of what eventually becomes Hieroglyphics. Their use continued through Old, Middle, and New Kingdom Egypt and into the Hellenistic period, as evidenced by the Rosetta Stone, which itself is only a portion of a decree announcing the coronation of King Ptolemy V and the establishment of his imperial cult. Hieroglyphics were used after the Hellenistic period as well, as the depiction of the Roman Emperor Trajan at the Dendera Temple complex attests. In other words, Hieroglyphics have a history of use as a writing system spanning over 3,000 years. So, with a history of use that long, when did Hieroglyphics fall out of use, resulting in the need of something like the Rosetta Stone to actually decipher them, and which only contains about 15 lines of that script? And why did they fall out of use? To actually answer this question, we need to understand that there was not just one writing system in Egypt. In Classical and Late Antiquity, Greek and Latin were employed, and eventually so was Coptic, a system of writing designed for the Egyptian language that was based on the Greek alphabet. Alongside these systems were the Demotic and the Hieratic scripts which recorded Egyptian, and of course the famous Hieroglyphics. In other words, by Late Antiquity, there were six scripts being used. These writing systems, in the context of Egypt, had different functions which evolved over centuries, or in the case of Hieroglyphics Hieratic developed out of Hieroglyphics and began to be used for administrative purposes and record-keeping during the 3rd millennium BC, while Hieroglyphics became something of a literary script. It was employed by priests, becoming formalized, and while it was sometimes used on documents, more often than not it was used for carving on temples or other stonework. Specifically, by the time of the Classical period, the style of writing for Hieroglyphics that was used was Middle Egyptian. Essentially, Egyptian as it was spoken and written between about 2000 BC and about the end of the Bronze Age around 1200 BC. That something like this was done is perhaps not all that surprising. There are plenty of examples of older forms of a language being preserved as a literary and an educational language, with the style of Greek used by Thucydides, for example, being taken as the standard in Greek and Roman history writing hundreds of years after that historian lived. In this context, Hieroglyphics was more of a formal script, while Hieratic formed the everyday functional writing system, a division which continued during the Persian period of rule in the 6th and 5th centuries BC, and this system then continued into the Hellenistic period during the Ptolemaic dynasty. But here we get a new writing system, Demotic, which began to appear probably around 650 BC in southern Egypt. During the Ptolemaic dynasty, which ruled Egypt between 305 and 30 BC, Demotic, Hieroglyphics, and Greek were all employed, as can be seen from the Rosetta Stone. After Mark Antony and Cleopatra lost the Battle of Actium in 31 BC, and following the subsequent defeat of their forces at the Battle of Alexandria the next year, Egypt was incorporated into the Roman state in 27 BC, where it became the In Roman Egypt, the Egyptian language predominated out in the country, but in the cities, Greek and later Latin were primarily spoken, so the employment of Greek and Demotic continued, and after a time, Latin was also thrown into this mix. Augustus and the Roman administrators after him fundamentally altered the religious landscape in Egypt, and ultimately it's this which led to the decline of Hieroglyphics. In Egypt, the Roman emperors became Roman pharaohs, and the imperial cult was incorporated into the system of Egyptian temples, where the emperors were worshipped alongside traditional Egyptian gods. But in the process, Roman land reform gradually removed the economic prerogatives and vast land holdings of these temples, something which stood in stark contrast to how things worked under the Ptolemies, who had generously funded these temple complexes. Some funding for the temples did continue through the Roman era, but the civil wars and other periodic disruptions interrupted the flow of revenues to these temples, further exacerbating economic problems associated with monumental construction, its upkeep, and the practice of the imperial cult. It's probably due to this economic decline that the knowledge of Hieroglyphics began to suffer. We still have plenty of evidence for the use of Hieroglyphics, and they are still written in Middle Egyptian, but mistakes began to creep in during the Roman period. These grammatical and syntactical errors began in the late 2nd century, and by the 4th century AD, the evidence of Hieroglyphics begins to sharply decline, and what was written tends to be relatively short. Demotic also declined during the Roman period, and because Greek and Latin were so heavily used in cities, the use of the alphabet began to be incorporated into Demotic, eventually leading to the development of Coptic, alongside an increasing prevalence of Latin and Greek in Egypt, well beyond what was previously used for administrative purposes. Into this economic decline of the temples, two other things were mixed in late antiquity. The first is a rise in what is basically best understood as new forms of Romano-Egyptian magic and magical thinking. Hieroglyphics and other aspects of Ancient Egypt became semi-Romanticized, and we begin to see a proliferation of magical texts coming out of Egypt as well as magical artifacts, and older Hieroglyphics began to be pointed to as examples of sacred writing. The second thing is the rise of Christianity. There is not really a neat division between these three occurrences, the decline of temples, the rise of magical thinking, and the advent of Christians. What's important, though, is that it's in this context, magic, that early church fathers in Egypt came into contact with and their use as magical symbols and charms. That interaction is not, contrary to what you may sometimes hear, an entirely negative one. In other words, there's not a strict division here, because late antiquity sees Christians employing symbols like the cross and bible verses as charms or symbols of their faith and hieroglyphics were used in the same manner by pagans. But while some Christians saw evidence of paganism and eventually, what we would understand, to be devil worship and demons, some of them also saw semi-Christian imagery in the form of the Ankh. And the reverse is also true, with pagans seeing similar things in Christian imagery. In 391, under Emperor Theodosius I, all pagan temples were closed and we see in this the culmination of a battle between pagans and Christians over religious symbolism and religious space. The use of hieroglyphics had been on a steady decline for about two centuries by this point, and now with funding officially pulled from all temple complexes in Egypt, the use of hieroglyphics officially came to an end with the last known inscription being done in 394 AD. But Egyptian did continue to thrive in the form of Coptic. Christianity did not so much put an end to the use of hieroglyphic writing so much as it just oversaw the culmination of a decline in the script. This last inscription, dating to 394, was done at the temple of Philae, believed to be the burial site of the god Osiris. And while the temple actually continued to function as a site of worship for Osiris, it was formally closed in 435 under the orders of the Emperor Justinian. There is, however, one text from the middle of the 5th century, known as the Hieroglyphica, which claims to be a translation of hieroglyphics into Greek, but already by this point the knowledge of the writing system was so decayed that the text makes basic errors in trying to explain the script. And with that, the knowledge of hieroglyphics became essentially lost. And although the symbols would be incorporated into occult and magical thinking over the coming centuries, or would otherwise simply be appreciated for their artistic style, that is all they would remain until the discovery of the famed Rosetta Stone in 1799.