Master's student in classics and ancient history here-- I would venture to say that orientalist ideas have at least something to do with it. I deal with nineteenth-century nationalism a lot when reading major historians' works from that time period. Of course Edward Said wrote the landmark study Orientalism that called them out on it, but it was and still is highly controversial. Mesopotamia and Egypt were of course considered eastern and are still mentioned and studied (although not near as in-depth as Greece and Rome), but I know I myself had never dealt with Indus Valley civilizations until my undergrad ancient history courses.

For example, right now I'm working on a term paper about Sol Invictus and astral religion in Rome up until late antiquity (which I know isn't necessarily relevant to your question), but many scholars (Wissowa, etc) will downplay or deny "eastern" influence, even altering or omitting the ancient sources, and it becomes apparent that they just didn't want their western society to have been shaped by anything other than the westerner's supposed intellect and ingenuity. One's nationality was fundamentally tied to one's ethnic value, and although we're a hundred years removed, I'd guess that sort of mentality echoes a bit as historical scholarship tends to move a bit lethargically.

There are probably a million different sides and answers to this (such is the study of history), but I simply mean to make a suggestion from my point of view as a classicist in training.