IMDB lists one possible explanation based on the poor geography knowledge of the screen writers. But it's careful to list it as "unconfirmed".
An unconfirmed explanation of why the setting was changed from Maine to Oregon is that one of the screenwriters misunderstood the novella's reference to "Portland" being nearby.
Another possible reason, and now I'm guessing, is this: the production company had chosen Oregon as the location ( https://www.imdb.com/title/tt0092005/trivia?item=tr2431593)
for other reasons (i.e. financial reasons) and worried that viewers would complain that the landscape (mountains, vegitation, architecture) doesn't look like Maine. Saying that the story takes place in Oregon removes such risks.