The americans LOVE shows that run and run. By that I mean the advertisers and the channels themselves.
Shows that run and run mean loads of advertising, loads of sales round the world, and loads of DVD sales afterwards.
In the UK shows like Fawlty Towers have 2 series of six shows then stop while at the top.
In the US shows like Friends have hundreds of episodes and try to go on as long as they can.
I remember Twin Peaks many years ago. Should have been a great short series with some strange goings on. As soon as it became popular that dragged it out to become almost like a soap opera.
I bet if they remade "Gone with the Wind" for TV in the US it would become a 100 part "mini series"
(I know some UK shows go on and on like "Last of the summer wine" but I think that is the exception rather than the rule)