From what I hear, there were a few factors: (1) it was fairly expensive to produce, and (2) there was a new head of the network, who didn't consider DLM "his show", and wanted to put on shows developed under his watch.
Maybe ratings, but I'm not sure what kind of ratings the show got. I'm pretty sure ratings started off strong, but over the course of the show, I don't know what levels they got.