Because Ancient Egypt is directly in the Classical line of Western civilization (see the Ptolemies, Hellenistic culture, the Rosetta Stone), and the medieval Serbs, interesting as they may be, are not.
They could probably come in in a survey of medieval culture, but what specifically was their contribution to medieval scholastic thought?
You can't study everybody, so historians tend to concentrate on the main trunk of the tree, not offshoots and remote twigs. Unless of course you want to make a study of a particular group out of personal interest (my ancestors the Highland Scots are pretty darned far off the beaten path, but I took Gaelic just because.)
The stopped the Ottoman invasion of Western Europe at the Battle of Kosovo and later, Serbian mercenaries fighting for the King of Poland relieved the siege of Vienna by the Turks.