Final answer:
The Eleans likely boycotted the Isthmian games due to their political grievances against Athens, which had imposed strict control and extracted wealth from member cities of the Delian League, including the Eleans themselves.
Step-by-step explanation:
The Eleans boycotted the Isthmian games primarily due to political conflicts and unrest in Ancient Greece, especially related to the actions of Athens.
During this time, Athens had become increasingly authoritarian over other Greek city-states it had ostensibly partnered with in the Delian League.
The league, initially formed as a mutual defense pact against the Persians, had morphed into an empire controlled by Athens, which enforced new laws, taxes, and political authority over its members.
Cities that were members of the Delian League, such as the Eleans, might have opted to boycott events like the Isthmian games as a form of protest against Athens' domination and exploitation of the league for its own benefit, which included funneling wealth to finance its grandiose building projects and the splendid age of Athenian democracy and philosophy.
Athletic events in Ancient Greece, like the Isthmian games, were not just sports competitions; they were deeply intertwined with politics and Greek identity. Boycotting such events would have been a significant statement of dissent and an attempt to resist Athenian hegemony.