“Unwilling” makes this wrong. We don’t know that Jewett was not willing to feature children and religious themes as much as other domestic novelists. She may have been just as willing, but chose not to make those features as prominent for artistic purposes.

The author doesn’t try to explain why Jewett focuses on rural instead of urban concerns.

The author tries to explain why Jewett wasn’t constrained to feature children and religion as prominently as other novelists were. Jewett’s conception of fiction as pure art meant she didn’t have to try to educate readers about domestic duties or religion. Other domestic novelists, because they conceived of fiction as having the purposes of acting as a child-rearing manual and tract on Christian duty, felt constrained to include more child-rearing and religious aspects to their works.

The author doesn’t try to explain why Jewett focuses on women and their concerns.

“Unable” makes this wrong. We don’t know that Jewett was not able to feature children and religious themes as much as other domestic novelists. She may have been just as able, but chose not to make those features as prominent for artistic purposes.