It's ratings were stable for a 20 year old show that kept being moved around. They should have gave it a final 13 episodes to"beat" the season record. Then launched LA midseason.
Dick Wolf claims that was always the plan -- shortened Season 21 to get the record and end the series proper, then launch
Los Angeles as its replacement in the spring.
Of course, then NBC did a 180 and canceled L&O during the filming of "Rubber Room," the Season 20 (and ultimately series) finale. It's a happy accident that the episode works pretty well as a series-ender, but the sudden cancellation forced the development of
Los Angeles to get rushed, which is part of why the pre-retooling episodes are so goddamn bad -- the show got moved into a September 2010 premiere slot despite not even having a pilot script written or filmed, and they didn't even start filming the pilot until the middle of August.