Tribune Media Co. terminated its merger agreement with Sinclair Broadcast Group and sued the rival TV-station owner, alleging it failed to make sufficient efforts to get their $3.9 billion deal approved by regulators.

Last month Federal Communications Commission Chairman Ajit Pai said he had serious concerns about Sinclair’s submissions as part of the agency’s review, and sent the matter to an administrative law judge, a severe blow to the merger’s approval chances.

The...