The ways of Japanese TV drama producers must be as mysterious to their Hollywood counterparts as the statues of Easter Island.
Whereas Hollywood prefers to spin out hit shows to infinity, while raking in millions from syndication — "Seinfeld" forever! — Japanese drama shows usually have limited runs of 11 episodes or so, no matter how stratospheric their ratings. They can have profitable afterlives on DVD and other media, but not the sort of syndicated eternities enjoyed by the most popular U.S. shows.
Japanese producers have explained this difference to me by saying that, for local audiences, dramas are like news — and thus naturally have short life cycles.