Why did the kingdoms in pre-classical Greece collapse?
The major kingdoms that existed in Greece before the classical period were called the Minoan and the Mycenaean cultures. Many books exist on the subject of the rise and fall of these civilizations. As only limited written materials survive from these periods, much of the information we have about the growth and collapse of these civilizations is still somewhat speculative.
The first major civilization of Greece was known as "Minoan" after the legendary King Minos of Crete. This civilization was a thalassocracy centered on Crete with strong connections to other Aegean islands and flourished from 2600 to 1400 BCE. At approximately 1450 BC, Minoan civilization was disrupted, possibly by the volcano at Thera, and possibly due to other causes, and then the major Cretan palaces were conquered by the Mycenaeans from mainland Greece. Crete participated in the general collapse of Mycenaean civilization ca. 1100 BC.
The Mycenaeans were a Greek-speaking civilization living in highly fortified cities in mainland Greece flourishing ca. 1650-1100 BC. The reasons for the collapse of their civilization may have been a combination of invasions (such as the "Dorian invasion") by external enemies and internecine strife. There is strong archaeological evidence for destruction of the palaces by some form of warfare, but no written evidence to clearly pinpoint the identity of the combatants.