Budget concerns on the side of BBC which I think is very wickity-wack ridiculous unless limited series lang ang ginagawa nila. In fairness for BBC. Medyo, to be fair, ambisyoso din sila gumawa ng mga in-house shows: Casualty, Holby City, EastEnders, Skins at Waterloo Road. If ratings increase for the next season, BBC could possibly consider commissioning more than 3 episodes per season (which was the case for some of their long-running shows).
Torchwood was on the verge of cancellation but because of the sudden ratings growth during Children of Earth, they struck a deal with Starz in a co-production venture.
As for CBS, mataas ang standards nila when it comes to ratings performance for all their shows. Usually they would cancel at least 1 new drama and a 1 veteran drama per season (as with this season's case: CSI Miami, AGM and Unforgettable, the season before: Medium and Miami Medical, and before that: Cold Case and Three Rivers). We'll see how Elementary would fare this fall.