Final answer:
The Isthmian games were dedicated to Poseidon due to his significant role as the god of the sea and other domains.
Step-by-step explanation:
The festival for the Isthmian games was dedicated to Poseidon, the god of the sea, earthquakes, storms, and horses. In Ancient Greece, athletic events were significant for displaying physical prowess, and the Isthmian games were one of these key athletic festivals.
Poseidon was an immensely important deity in Greek mythology, as evident from various myths that involve his intervention in human affairs, such as mediating contests and punishing those who defied the gods, like Queen Cassiopeia when she compared her beauty to that of the sea nymphs, daughters of Poseidon.
Additionally, the myth of the contest between Athena and Poseidon over who would be the patron deity of Athens emphasizes the importance and influence of Poseidon. Ultimately, though Athena won this particular contest, the city of Athens housed a temple that honored both deities, with evidence of their contest, such as an olive tree, a salt-water well, and marks from Poseidon's trident.
These elements illustrate how intertwined the worship of both Athena and Poseidon was within the culture of Ancient Greece, and why Poseidon, with his vast domains and powers, would be honored specifically at the Isthmian games.