The GTA 5 Trevor DLC was scrapped because GTA Online became a "cash cow" and the team prioritized resources on its development and supporting Red Dead Redemption 2. Former Rockstar senior camera artist and virtual cinematographer, Joseph Rubino, revealed that the decision was made as GTA Online's success made it difficult to argue for a standalone DLC.
In the scrapped GTA 5 Trevor DLC, Trevor Phillips was supposed to play the role of an undercover agent working with the FIB (Federal Investigation Bureau). The storyline would have followed Trevor going undercover as a secret agent, with the finale taking place in outer space2.
GTA Online's success led to the cancellation of the GTA 5 single-player Trevor DLC. The online mode became a "cash cow" for Rockstar, and the company shifted its focus to multiplayer mode, prioritizing resources for GTA Online instead of the standalone DLC. Some elements from the planned DLC later appeared in online content.