truepenny has said it better than I could really, but I will just leave you with my own personal horsey simile.
If you compare the three types of writing to a three day event, poetry is dressage, the short story is show jumping and the novel is the cross country. Poetry requires utter precision and technical skill and not everyone appreciates it! Short stories are a little more forgiving (though not much) and they have to stay very focused on one area. With novels, you cover much bigger distances and as well as coping with technical obstacles along the route, you have to think a lot about rhythm and pace and you need the stamina to get to the end.