That's the whole point of the show. The hospital as a whole deals with many other medical aspects, however Dr. House specializes in the hard to figure out (or off the wall) diseases and illnesses. It is not a show about the entire hospital and what they deal with, it is a show about Dr. House. Plus how boring would it be to sit there watch him diagnose someone with a very common disease for an hour? He would know right away, there would be no mystery, no complications, no story line.