That would have to do with the Eastern Roman Empire being a footnote in World History books, because people would rather read about the Dark ages of Europe, or the Arab conquest of the middle east and North Africa. But those men from Constantinople where a proud and brave people, the last vestige of Civilization in a time of barbarism, strife, and plagues, Byzantines stood fast, and held onto Civilization. By 1000AD Europe came out of the Dark Ages for the most part, and looked to Byzantine's for knowledge. The reason they feel was internal strife, and overspending, the same problem that collapsed classical Rome 1000 years earlier. Had they better managed there Empire, the Turks wouldn't have maintained a foothold, but due to poor management and strife inside there own empire they where tore apart by the Turks.
It is true that the Byzantine Empire is overlooked, and that it had contributed much to Europe. I, myself, find it very sad above all else that history has labeled this empire incorrectly. Historically, nobody called them Byzantines or the empire Byzantine. That is a modern label given because their capital was built in a region we call Byzantium. The Byzantines addressed themselves as
Romaios, the Hellenized form of
Romanus, or Roman. They viewed themselves -legitimately- as the Romans, and as such, we forget just how much they earned that label. While the rest of Europe forsook bathing and education for violence and squalor (not voluntarily, of course) the
Romaioi kept Roman tradition alive with all the same cultural prowess, construction ability, and military organization of the original Empire on their side. While they did share the name and infrastructure of the old Romans, they maintained a very Greek attitude and Hellenic culture; they spoke Greek, valued personal hygiene, and wrote and thought like Greeks. For this reason, the Byzantines are a civilization all their own, the ultimate culmination of Greco-Roman culture.
As a side note, I love how the Byzantine Empire was a derivative Civ of Rome in Civ4: BtS.
Post-Script: I just read Dachs' posts and I agree with his side of the Justinian argument. When one looks at the original Roman Empire, and how it expanded, Gaul was only added because Julius Caesar need to conquer a large swathe of land for political reasons. Other than -perhaps- some mineral resources, Gaul and Northern Spain lack much of anything valuable to a trade power like the Roman Empire. I think that Justinian had conquered enough territory for one life. If anything, I think he would have spent the remainder of his life ensuring that the provinces he conquered were secure and ready for his successors. The Mediterranean is indeed a great font of wealth, and with such wealth there is little need to conquer more: from the Nile one attains food, from Athens knowledge, and from Rome and Carthage the boundless fruits of international trade. On top of all those lovely prizes sat Constantinople, the jewel of medieval Europe, a little slice of Roman heaven, an oasis amongst the shattered remains of a once-great empire.