The (U.S.) commissioning departments pay very close attention to the ratings because of the advertising revenue.
An American series last for 26 weeks, often broken up into two lots of 13 episodes. If the money-figures don't stack up it'll often end after week 13 leaving those who were enjoying it deeply dissatisfied - i.e. FlashForward.
Conversely, a storyline which ought to be resolvable in 13/26 weeks, will be stretched beyond its natural limits when it proves to be a hit with the viewers leading to tedious, unbelievable twists, etc. which are all to do with chasing the advertising dollar rather than providing the viewer with an enjoyable experience - Lost, Heroes, etc.