One reason could be to protect the creative integrity of the game. The writer may have had a specific vision for the story, and by defending it, they are ensuring that players understand the intended narrative. For example, perhaps there were elements that players misinterpreted as being out of place or inconsistent, but the writer can clarify that they were intentional parts of the overall design.
The writer might be defending the story to counter negative criticism. Maybe some reviewers or players thought the story was too complex or not engaging enough. By defending it, the writer can point out the nuances and depth that they believe exist in the story. For instance, the character development arcs that might not be immediately obvious but are integral to the overall plot.