The GTA 5 story DLC was canceled because GTA Online emerged as a "cash cow" soon after launch, making it hard to argue for a standalone DLC's success. The decision was a business one, prioritizing the more profitable online mode over the single-player expansion.
Joe Robino was a senior camera artist and virtual cinematographer at Rockstar's New York office from 2010 to 2016. He worked on games like Grand Theft Auto 5, Red Dead Redemption 2, and Max Payne 3. He was also one of the main editors and camera artists for a standalone GTA 5 story DLC that never came out.
GTA Online's immense success and popularity led to the decision to cancel the standalone Grand Theft Auto 5 story DLC. The revenue and player engagement generated by GTA Online made it difficult for the company to justify investing resources in a standalone DLC, which might not have performed as well.