This Boston Globe article says that filing has now closed for candidates running in Massachusetts primaries this year, and in six of the state’s nine U.S. House districts, no Republican filed.
Although the article is fairly lengthy, and tries to explain why this happened, the article never mentions the real cause, which is that Massachusetts ballot access for primaries is the most restrictive in the nation. Massachusetts requires 2,000 signatures for a candidate to get on a primary ballot for U.S. House, more than any other state (for primaries).