These are two different things.
The cancellation of some shows in the way Aaron mentioned is a way of a new boss to show "who's in charge now". It is stupid and ignores the audience's will and is disrespectful towards the writers and actors who did nothing wrong.
The other thing is a general decline in quality of sitcoms. Maybe this is caused by the audiences fondness for cheap but flashy reality soaps such as the "talent" shows or things like "Snog, Marry, Avoid", "Big Brother" etc. It's a hard time for subtle comedy writers to compete with that sort of rubbish. And the TV bosses are confused...they don't know what to commission to please the audiences. New shows that are sexy, cool (progressive filming style or those targeted at teens and twens) and with a dose of reality (think of the bloopers in Mrs. Brown's Boys)? Or back to old school (The Royal Bodyguard)...without checking first if the scripts are any good?