Possibly the biggest reason is the state of sources: We have Herodotus for persian wars, Thukidydes and Xenophon for Peloponesian war and 1 half of 4 cBC. But after that only scraps until Alexander. Those end on his death or at last around 300 BC, and then next major work is Polybius covering around 260-160. This means that for example Philip II before Vergina finds was seen through eyes of Demosthenes - as brutal barbarian, alawys drunked and destroing Greece for the pleasure of destruction. This wiew dramaticaly changed after tomb atributed to him with all that gold and great art was found. Yet still he was shown from Dhemostenic perspective in movie Alexander.

On the other hand there are quite a few sources concerning romans. Most of them were writen much later and obviously try to show how inevitable was roman rule to the world, and how Greeks (Carthaginians etc) were unable to rule themselvs, how evil they were and how good the romas were to invest time in converting them to good roman ways.

School programmes were writen mostly many years ago, possibly in mid XX century. At that time it was still considered among scholars that hellenistic period is the age of fall, and is not worth studying. This means they concentrated on classical Greece, both historian and archeologists.

for example as far as macedonia is considered, before revealing "vergina tombs" and especially "tomb of Philip" it the 70s only few archeologists were digging in Macedonia. After that finds amount of works, and so obviously publications skyrocketed.

there are still reservations about hellenistic period though. Many still see it only as a period when Rome was growing to power. So no matter that we found many reasons to believe that as far as greeks are concerned the hellenistic period is just the opposite as was thought (so as the era of great technological ad scientifical developement) the old view still holds well and especially in school programmes that nobody bother to change.