I have to wonder about the underlying assumption here.
Zeus is the standard Indo-european chief God.
The Proto-Indo-European pantheon includes a number of securely reconstructed deities such as *Dyḗws Ph₂tḗr, the daylight-sky god; his consort *Dʰéǵʰōm, the earth mother; his daughter *H₂éwsōs, the dawn goddess; his sons the Divine Twins; and *Seh₂ul, a solar goddess. Some deities, like the weather god *Perkʷunos or the herding-god *Péh₂usōn,[note 2] are only attested in a limited number of traditions – Western (European) and Graeco-Aryan, respectively – and could therefore represent late additions that did not spread throughout the various Indo-European dialects.
Now, certainly it's possible for pantheons to rearrange, the Norse cognate is Tyr, of all people!!
It's entirely possible that Poseidon was a pre Indo-european God, there seems to be a lot of dispute on his origin, and that of his name.
Remember that the original Achaians were barbarian invaders into civilized lands, it's not surprising that they'd pick up local gods.
Given, however, that the later Greek invasions (Doric, ionic, aetolian) were barbarian invasions into the barbarous remains of a collapsed civilization, it's not surprising that they would keep more of their Indo-european roots, including the chief position of Zeus.