Unlike the several DC shows before it, Constantine did not get a full season order. It must be remembered though, that the show has not been cancelled. Executive Producer David S. Goyer noted that in a recent interview
“It’s not dead, at all, it’s solidly in contention for a second season of NBC, which is still supporting it,” he said. “Constantine got caught in the midst of a change that is happening at the networks. It gets big numbers on DVR and online views, but it is sort of a tweener that doesn’t fit the traditional viewing box at the big networks. It’s not State of Affairs, it’s just not that kind of show, but NBC likes it. It’s frustrating what happened because creatively we’ve really started to hit our stride and people are responding to that. If I were a betting man, I would say there’s an excellent chance for a second season.”
There you go. As long as you keep watching, it’ll continue.