There are a handful of reasons that Egypt is given more attention than sub-Saharan Africa. The most prominent is by virtue of what history is, the study of things that have been written down. We define "prehistoric" vs. "historic" by the advent of writing, so it makes sense that writing would be important to the study of history. Egypt has written history, while most of sub-Saharan Africa maintained the use of oral tradition until the spread of Islam, and even then they did not write down their oral history very often. It is only recently that historians have given any credence to oral history. Prior to this shift in attitude, it has often been the case that we consider oral history to be unreliable, and thus it is disregarded. In the absence of a written history, European historians essentially treated sub-Saharan Africa as if it had no history. This shift in attitude is still ongoing, but with the vast collection of Egyptian writing, especially on the walls of temples, it never suffered that fate. Even when we could not read it, it was impossible to argue that it was not there.

Similarly, it would be impossible to look at the building projects of Egypt and argue that there was no history in the region. Egypt has impressive monuments that draw a great deal of attention. The pyramids, the great sphinx, Karnak temple, and the complex at Abu Simbel still astound and fascinate people to this day. Speaking for myself, these grand structures were the inspiration that started me studying Egyptology, and it has been my experience that I am far from alone in this. While there are some pyramids at Meroe, a lot more attention is paid to Egypt because the monuments there are older and larger.

To address your question of disassociation of Egypt from the rest of Africa, that is because Egypt was far more of a player in the political and historical situation of the Mediterranean world than in the rest of Africa. Egypt had interactions to the south in modern Sudan and to the west into modern Libya, but otherwise Africa was largely ignored. They extended their influence over Sinai and into the Levant far more frequently. Egypt took an interest in the conflicts between Assyria and Babylon, and in the rise of the Persian empire. Egypt was a vital part of the legacy of Alexander of Macedon, and under Ptolemaic rule it was a Hellenic kingdom. Arguably the most famous Queen of Egypt, Cleopatra VII, was Greek, and her relationships with Julius Caesar and Marcus Antonius are rather infamous, even today. Even prior to Hellenistic rule of Egypt, the Greeks and Egyptians were well known to each other. It is theorised that the Greek god of medicine, Asclepius, originated in the Egyptian architect-turned-deity Imhotep. We know that the two gods were treated as being the same, in much the same way that the Roman Jupiter and Greek Zeus were the same. In the years since Egypt was absorbed into the Roman empire and now, the country has been associated with the Byzantines, a series of Muslim caliphates, and the Ottoman Empire, all European/Mediterranean powers. This is the reason that many cultural scholars discuss the MENA (or Middle East and North Africa) region, as the languages, customs, and cultures of North Africa is far more closely related to the Middle East than to sub-Saharan Africa. This is not so much ethnocentrism as it is the acknowledgement that Egyptian history is tied to the history of the Mediterranean much more so than the history of the rest of its continent.

You have to recall that the modern continental map is exactly that, modern. The Egyptians would have considered the Levant part of their lands, while modern South Africa, as an example, would be too far away to be of much concern to them. While the Sahara is not the impenetrable barrier that many imagine it to be, it must be understood that Egypt was much more of a factor in the history of the Mediterranean than it was in the history of Africa.

If you want to do more reading on this subject, I highly recommend pretty much anything by Professor Bob Brier, but most notably Egyptomania: Our Three Thousand Year Obsession With the Land of the Pharaohs, which spends a good deal of time discussing why Egypt is fascinating to us, as well as the history of the land itself.

The disassociation of Egypt from the rest of Africa is a left over of colonial thinking. There is no real reason to disassociate Egypt from the rest of Africa.

Also, the concept of lumping countries into "sub-saharan Africa" comes from a western colonial mindset as well. African countries are culturally and historically distinct and diverse from one another. Even the term doesn't make sense; "sub-sahara" means "under the Sahara". There is no country under the Sahara, there are only countries south of the Sahara.

As an Ethiopian, I see Egyptian history in the same way I view Ethiopian history or Zimbabwe's history or Malian history; all of them are different and distinct but they are all equally African. Egypt is no exception.

I would love to elaborate more on the matter if you have any specific questions you'd want to ask.