The reason Enterprise got cancelled isn't necessarily due to low ratings, though that in part, had alot to do with it. It is advertising, which is where the networks get the money from, to buy shows from the studios.
For example, Paramount says, "We have a show idea." So they pitch this idea around to a bunch of networks with a ballpark estimate of 30 million dollars a season. Now a network pays the price tag of $30 million, in the hopes that advertising for the show with net them a certain amount above what they originally paid for the season. i.e. 60 million dollars in avertising sales over the course of the season.
However, if the show fails to net them a profit over the course of a season they may put it on the chopping block. Advertisers probably just didn't believe in the production, thinking that the viewers aren't diverse enough to be worth reaching.
America as a whole seems to have gravitated to the Idols and the next top model shows which is where the advertisers are now sending their millions. Not necessarily because the ratings are super high, or because enterprise was good or bad, but because the crossection of viewers is higher and more pliable, therefore a better investment.
As I am unsure how a studio fits into the order as where it's profit comes from, I'd appriciate anyone that might shed some light on that. I just thought I would point this matter out since everyone seemed hell bent on slamming enterprise, when really all the nay sayers that dare call themselves fans of Star Trek killed it by themselves.
-Rance-