Many who are not familiar with Egypt's history and culture wonder why Egypt does not speak Egyptian today and not Arabic. This article tries to answer this question for those who ask it.
The New Kingdom: The Last Empire
Egypt's last purely Egyptian great empire, known as the New Kingdom, lasted from 1567 B.C. to 1085 B.C. During this period Egypt dominated a lot of its neighbors, such as Nubia, and Syria. This was achieved by various means, such as Thutmosis military campaigns, Ramses II's peace treaties, and Amenhotep III's diplomacy via marriage. The riches of Tut-Ankh-Amen are from this period, as well as the religious revolution of Akhnaten, the heretic monotheistic pharaoh.
Even then, the language of diplomacy was not Egyptian, but rather cuneiform. Baked clay tablets from Tell El Amarna are written in cuneiform script, and form the letters sent from/to vassals in Syria.
Decline and Decay
Egypt's decline started after this period, when priests took over, and centuries of weakness and decay ensued. During that time, most of the royal tombs were looted, either bypriests recovering the riches of long gone by pharoahs to prepare thenew ones, or by robbers looking for gold. Egypt came under successive rule from foreign powers, such as the Nubians, the Libyans, the Persians, and the Assyrians.
An interesting observation is that Egypt was never ruled by a native Egyptian from the time of Nectanebo II, c. 343 B.C. down to 1952 C.E. when Mohamed Naguib came to power! That is about 23 centuries!
The Hellenic Era
Then after Alexander conquered Egypt in 323 B.C., the era of Hellenism started, where Egypt was a Greek cultural center. Egypt saw a renaissance, but not an Egyptian one, it was purely a Greek one. The cultural centers of old in the south never recovered their past glory as cities of pharoahs. The Egyptian language fell into disuse: apart from use in liturgy by some priests and a dialect for the peasants. It was not the language of the state, the language of commerce and trade, nor the language of learning.
Roman and Christian Eras
When Christianity came to Egypt, Egypt was already in decline, having came under Roman rule, and later under Byzantium. Egyptians, like others in the area, never liked their Roman overlords, in fact they loathed them.
Under Christianity, there was persecution against the pagans, such as the mob killing of the philosopher and mathematician Hypatia of Alexandria, and the destruction of whatever remained from the Library of Alexandria.
The Arab and Islamic Era
So, Egypt was ready for another era of greatness, but as was true for 1.5 millenia, it would be under a foreign power. The Arabs came in at the invitation of the Egyptians, who loathed the Byzantines. This was a recurring theme in early Muslim conquests, such as that of Julian, the ruler of Cuetta in western North Africa and the supporters of Witiza summoning the Muslim Arabs and Berbers to conquer Iberia (al-Andalus).
Arabic as a Language
As for the loss of language, Arabs/Muslims never enforced Arabic on the population. In fact the process of changing from a minority language to the language of the majority took six centuries to happen.
The simple explanation is that Arabic, being a semitic language, was close to spoken Coptic, and other Semitic-derived languages. This is why areas where a semitic language was spoken (e.g. Nabatean, Syriac, Assyrian, and even Berber) were all easily supplanted by Arabic, while in areas where no semitic language was spoken (e.g. Persia) the native languages persisted.
This is why we see that in Central Asia, India, Malay and Indonesian Archipelago, West Africa, and East Africa, Arabic never took hold as a majority language, despite of a significant portion of the population being Muslims, and a majority in most cases.
Hence, Coptic, the successor to Ancient Egyptian, was relegated to becoming a liturgical only language, and Arabic became the majority language for Muslims and Copts alike.
Greatness and Decay, once again
Egypt was once againt destined to become the leader of a great civilization. The Fatimids build Cairo, and Al Azhar university. Saladin brought the downfall of the Fatimids, but he and his successors, continued to make Egypt a great hub of civilization and learning. After the fall of Baghdad in 1256 C.E. to the Mongol hordes, Egypt became the greatest center in the region, under the Mamelukes. Only after the rise of the Ottomans and their conquest of Egypt in 1517 C.E. did the center of power and greatness shift to Istanbul.