Even though cancellation maybe be in Constantine near future, it could be a very dumb choice by NBC.
Hellblazer, or Constantine, is a very popular comic character to hundreds and hundreds of underground fans. The 2005 Keanu Reeves movie left a very bad taste in fans mouth. The movie was loosely based on the comic book and it's characters, and Keanu Reeves was the choice for Constantine no fan wanted. Now, skip ahead almost a decade and bring the Constantine franchise to television. This new version of Constantine is very true to the comic book and it's characters, like 90% dead on! Starring Matt Ryan, Harold Perrineau, Angélica Celaya, and Charles Halford. Constantine is very true to it roots, and is more than deserving than it's cardboard copycat competitors Supernatural and Grimm.
I'm not trying to bag on Supernatural, it's a good show, but what is also very interesting that 45% of Supernatural's material is very similar the Constantine's plots, storylines, and characters. I'm sure some hardcore Supernatural will not agree, but Constantine was created first.
So why would they cancel Constantine? Constantine is a high action, creepy, true to it roots show, with an amazing cast. I think if we want to the true answer we will have to ask the all incompetent suits at NBC.