It may not come as a surprise to anyone familiar with the ratings of the show, among the lowest for any primetime network show this season. The official word comes from Frank Spotnitz's blog:
It is with regret that I confirm "Night Stalker" has been canceled. While I’m disappointed the series has come to an end, I am enormously grateful for the experience and the opportunity given me by the network and Touchstone Television. I would like to thank our amazingly talented cast, wonderful crew, killer post-production department, terrific writing staff, producers and partners in crime Daniel Sackheim and John Peter Kousakis, the incredible Jana Fain, and all of you who watched and enjoyed the show.
It was a blast.
And that's all she wrote, folks.