I always thought Constantine was an odd choice for broadcast TV, given the darkness of its storylines and its central character being a classic anti-hero. It would have been far better suited to a network without censorship concerns. But on the whole, I think they did a great job with it. John was well cast and played, and most importantly, he was still an anti-hero.. The storylines, while cleaned up, still retained grit.
Why SyFy didn't jump at this I can only guess: money + full fall schedule. IOW, bad timing to shop a show to them.
CW seems to be doing alright with other DC properties, so it could be a good fit. I don't think they'll bite, though.
Dark horses: I wonder if he's tried talking to USA, Turner, or BBC? BBC produced several good supernatural shows- most, unfairly short lived, including one of the scariest shows I've ever seen on broadcast TV, Apparitions. They could do it.
...as long as they don't recast it with Jeremy Clarkson.