'Numerous warring petty kingdoms' isn't really a very accurate term. The Egyptian New Kingdom would never have been described as 'petty', and the Ptolemaic Empire held roughly the same territory. The scale of the warfare and politics here is made apparent by Diadorus Siculus' very short account of one of Ptolemy's campaigns.

'Seeing that Phoenicia and Coelê Syria, as it was called, were conveniently situated for an offensive against Egypt, he set about in earnest to become master of those regions.40 2 Accordingly he dispatched an adequate army with Nicanor as general, a man selected from among his friends. The latter marched into Syria, took the satrap Laomedon captive, and subdued the whole land. After he had likewise secured the allegiance of the cities of Phoenicia and placed garrisons in them, he returned to Egypt, having made a short and effective campaign.'

This area included modern-day Lebanon and about half of Syria: a pretty massive area with a significant and prosperous population. Yet it's little more than a footnote in this grand narrative. The Wars of the Diadochi splintered one of the largest empires in history into smaller empires.

The end of the Macedonian Empire was a relatively quick event, all taking place within 18 years. Another point worth noting is that the successor kingdoms remained part of a Hellenised world, a shared cultural sphere. The same cannot really be said of the kingdoms that succeeded the Western Roman Empire. England was essentially cut off, and had to be re-Christianised two centuries later. It was a similar length of time before the Visigoths converted from Arianism. 'Western Christendom' was gradually established by the nascent Papacy, rather than being a direct legacy of Rome.

Overall though, the reason these ends of empire are seen so differently is that one was quickly split apart by its own leaders, and the other was gradually chipped away, bit by bit, by invaders and mutinous allies. The term 'Dark Ages' is considered problematic, but you can certainly understand by English historians used it to refer to their own country. The Roman aristocracy wrote letters that then traveled hundreds of miles. They enjoyed steam baths and underfloor heating, having seafood delicacies brought up from the coast. The early Anglo-Saxon aristocracy were illiterate, and sent verbal messages no more than fifty miles. They lived in buildings made from sticks and poo, and their greatest luxury was mead. This stands in contrast to the aristocracies of the Diadochi kingdoms, who consolidated and prospered in the decades following Alexander's death.