Why is the story "The Celebrated Jumping Frog of Calaveras County" a frame narrative?

This humorous story by Mark Twain is considered a frame narrative because it consists of a story within a story. The outer story, the frame story, is the story of the narrator's interaction with Simon Wheeler. This story has its own conflict--the narrator vs. Simon Wheeler--and its own story arc. The inciting incident is where the narrator meets Simon Wheeler as a result of a friend's practical joke. The action rises when Wheeler backs the narrator into a corner and starts assailing him with his "monotonous narrative." The "inside story" that Wheeler tells is part of the rising action of the outside story. The climax of the outside story comes when Wheeler is called away, allowing the narrator to make his getaway, and the falling action is when he says his good-bye to the "good-natured, garrulous" old man. 

Within that frame, there is another story, and that is the tale told by Simon Wheeler. While Wheeler has his listener "blockaded ... with his chair," he tells the story of Jim Smiley, a man who was "always betting on anything." The rising action occurs as various exploits of Smiley are related, including his general proclivity for gambling, his wins and losses with his "fifteen-minute nag," his bull-pup Andrew Jackson, and finally his frog, Dan'l Webster. The incident of Dan'l Webster with the stranger brings this story to its climax as the stranger wins the bet by deceit. The falling action is when Smiley figures out the stranger has filled his frog with buckshot and goes after the "feller, but he never ketched him." 

From the resolution of the Jim Smiley story, the narrative returns to the outer frame story of the narrator and Simon Wheeler, feeding right into the climax of that story. Because it contains two stories in one, an inner story and an outer story, this story qualifies as a frame narrative.