From what I have gathered from various Japanese sources and Japanese observers...
A 2ch poster recently summed up in general what factor primarily determines the success of a television animation (anime) series in Japan:
1. Golden time (prime time) anime - television household audience ratings
2. Morning and late afternoon / early evening anime - merchandise sales (e.g., toys)
3. Late night anime - DVD sales
With a low, declining birthrate, an aging population which will likely shrink in the future, and the growth in the variety of media and entertainment, among other things, anime increasingly will take a backseat to news programs, variety shows, and live action dramas on mainstream television in Japan.
With few exceptions like Sazae-san and perhaps Chibi Maruko-chan, the elderly in Japan generally do not watch anime on television. Japanese teenagers and adults prefer to watch live action stuff (news, variety, drama, etc.). Children have been - and continue to be - the largest group of viewers for anime in Japan.
In Japan there are less children around today than in past generations. And Japanese children today, I have read, watch less anime than did Japanese children of yesteryear. (A Japanese source cites the Nintendo Wii as one cause.)
Japanese companies frequently target niche anime titles at male otaku - and increasingly at female fujoshi - because they can make money off them selling DVDs (and Blu-ray discs) and other merchandise (figurines, character goods, drama CDs, games, etc.).
Because of low, declining TV ratings, Meitantei Conan (Detective Conan aka Case Closed) and Yatterman for a while have been rumored of being removed from their golden time broadcast time slots on NTV, Mondays, between 7:00 PM and 8:00 PM, in the Kantou region.
Well, it looks like this will happen.
Beginning this spring in April 2009, Conan will move to Saturdays, 6:00 PM, while Yatterman will move to Sundays, 7:00 AM.
