[Nyororin]

I think it is a combination of two factors - the amount of money it takes to produce a new show in the US, and being sure to get that money back.

Japanese programs tend not to cost all that much to produce. Actors don`t really get paid all that much (Not even any more than a regular job for all but the most very famous). The production costs for set and effects are generally low as the settings tend to be fairly "normal", and not require CG or much travel. Because of the low cost of producing a new drama, instead of making one longer it is not all that much more expensive to make one after another.

When it comes to US programming, they aim high and spend a huge amount on production. Sets are often distant and exotic places, or they make it seem like they are by using a lot of complex effects. The pay for an actor on an average tv show is usually 10 times what it is in Japan. A LOT of money goes into making a new show, so there is a lot of risk when it comes to getting this money back. If a show flops, that is a LOT lost. So when a successful show comes along, instead of cutting it off in it`s prime (of money making) by setting the number of episodes ahead of time and upsetting all those shareholders who put out the money to start the show in the first place... They stretch it out to get as much out of it as possible.

It seems that with the US, when there is a successful formula, they stretch it out as long as they can. With Japan when there is a successful formula they remember it and recycle it in a different show later.
It`s all about how much it costs to start something new and how much of a return that will give.