Why was it cancelled...

It wasn't cancelled. I just watched the last episode this evening on Sleuth. There are a total of 98 episodes. I'm just sorry that it's all over.

The show was cancelled during the sixth season. That is why the final episode of season six ended the way it did, with Nicco being killed and Ali quitting the force. The producers planned to fight the network for one more season and eventually got their wish. However they did not know if this would happen while completing filming of season 6. That is why Ali looks at the camera and smiles in the last scene of the final season six episode. If it did end up being the last episode of the series, this was meant to signify that she couldn't stop being a cop because she cared too much about the work and the people who'd lost loved ones, and leave the audience with the impression that she took back her decision to quit and continued on solving cold cases, while at the same time leaving it open for her to be back on the force if it did come back.

A year and a half passed between the end of season 6 and the start of season 7. With the Nicco character having been killed off and Tamara Craig Thomas not interested in returning, the two new characters seen in season 7 were introduced.