why is the princess's internal conflict so central to the story?
The princess' internal conflict is central to the story, because the outcome of the story depended on her decision. How would she direct her handsome young lover? She has the power and control in the story to direct his fate; "possessed of more power, influence, and force of character than any one who had ever before been interested in such a case, she had done what no other person had done,--she had possessed herself of the secret of the doors."
Stockton's story asks the reader to examine the motives of the princess to determine the outcome of the story. The open ending leaves the outcome of the young hero's face undecided, so the reader must analyze the princess' internal conflict to come to grips with the resolution.
"The question of her decision is one not to be lightly considered, and it is not for me to presume to set myself up as the one person able to answer it. And so I leave it with all of you: Which came out of the opened door,--the lady, or the tiger?"
Stockton's ambiguous ending focuses on the internal conflict of the princess, creating debate among readers everywhere.