why was Zeus leader of the gods ?
although the question seems to be quite simple,it is quite complex, instead, because Zeus, whose name is connected with an ancient Indo-European deity named “Dyeus”, roughly translated as "sky"/"day"/"light" (as opposed to night and darkness), represents a transition from a Female-dominated pantheon ruled by a Mother Goddess [the so-called Potnia Theron (ancient Greek, Πότνια Θηρῶν, "The Mistress of the Animals", which is a term first used by Homer in Iliad 21. 470] to a Male-dominated pantheon where this god was originally worshipped as a weather god by the Greek tribes who came southward to Greece from the Balkans circa 2100 BC.
His main attribute was in fact the thunderbolt, as Zeus controlled thunder, lightning, rain and thunderstorms that he sent against his enemies, as we read in Homer's Iliad .
In short, I think that in a Male-dominated pantheon Zeus was considered the king of the gods because he personified the sky with all its atmospheric phenomena, especially the most horrifying events like thunder, lightning and thunderstorms that terrorised those primitive Greek tribes and made men feel very frightened, as they did not know the nature of those phenomena and then needed a supernatural explanation, i.e. a god to whom they could address to have protection against the severe forces of nature that they did not understand.
Generally speaking, in fact, Greek mythology derives just from this attitude of primitive men towards nature so that they needed to invent gods and goddesses that helped them to understand the surrounding world.